Item 1.  Report to Stockholders.

[Calvert Social Index Fund Annual Report]

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Dear Shareholder:

Stock markets ended the 2012 annual reporting period sharply higher as the Standard & Poor’s (S&P) 500 Index returned 30.20% versus 1.14% for the 12 months through September 30, 2011. While the two periods shared many commonalities--such as a softening of economic indicators, mounting concerns about sovereign debt in the eurozone, and relatively strong corporate profits--there were some key differences.

Financial markets around the world breathed a sigh of relief after the European Central Bank committed to saving the euro this summer and offered to support the debt markets for any euro member government seeking a new financial bailout. And while the disasters in Japan and strife in the Middle East and North Africa weighed heavily on investor confidence last year, uncertainty closer to home rattled investors this year, primarily as a result of the presidential election and looming “fiscal cliff” in January 2013 that could raise taxes for many Americans. The good news is both situations should see some resolution by the end of 2012.

The U.S. economy’s relative strength was also more evident in this annual reporting period. The divergence with foreign stock market returns widened as the S&P 500 Index more than doubled the 14.33% return of the MSCI EAFE Index for the 12-month period ending September 30, 2012. In the 12 months ending September 30, 2011, the S&P 500 barely eked out a positive return compared with -8.94% for the MSCI EAFE for the same period.

A Truly Uneven Recovery

Within the United States, the economic recovery was also uneven, with some consumers and businesses feeling left behind in the midst of an “improving” economy. For example, growth in northeastern metropolitan areas lagged, while metro areas out west, particularly those in natural gas or high-tech centers, rebounded strongly. ¹ Also, the housing market finally turned the corner, as national home prices rose 3.6% year-over-year for the past 12 months. However, they surged 9.4% in the western U.S. while notching up only 0.9% in the northeast. ² In employment, more than 70% of jobs lost in service industries during the recession have returned, but only 15% of jobs lost in manufacturing, construction, and other goods-producing industries have come back. The differences are stark even within industries, where retail general merchandisers such as Costco recouped 92% while department stores regained just 41% of lost jobs. ³ Overall, we remain cautiously optimistic about the economic recovery ahead. As both citizens and investors, we hope that government legislators return to the negotiating table after the election and give us all the gift of more certainty before the holidays.

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Making Strides in Board Diversity

Calvert continued its advocacy efforts in many areas, including board diversity. The United Nations Gender Equality for Sustainable Business Event in March marked the two-year anniversary of the U.N.’s Women’s Empowerment Principles (WEP), which were adapted from the Calvert’s Women Principles ^® , and emphasized the business case for promoting gender equality and women’s empowerment in the workplace. Calvert also joined the newly formed WEP Leadership Group to expand implementation of the Principles.

On March 21, we testified before the Senate Democratic Steering and Outreach Committee about the importance of gender equity in economic empowerment and job creation, highlighting research showing the inclusion of women in corporate management correlates with higher shareholder value and better operating results.

Creating “The Future We Want” in Rio

Finally, I had the pleasure of representing Calvert by speaking at several events at the Rio + 20 Summit about board oversight of sustainability, social enterprise and impact investing, the role of business in promoting gender equality, the business case for the green economy, and establishing value for natural capital.

The first Earth Summit in Rio de Janeiro 20 years ago created climate and biodiversity conventions and set the stage for frameworks to address global environmental degradation, climate change, and poverty. However, global economic uncertainty weighed heavily on the Rio + 20 Summit. While negotiations continued to advance toward 2015, we’re disappointed by the lack of concrete commitments needed for substantial progress on critical sustainability issues.

However, side events sponsored by private-sector and non-governmental organizations sparked many innovative initiatives and positive outcomes. A number of major companies made significant commitments on water, energy, renewable materials, and deforestation. Corporate and investor disclosure of environmental, social, and governance (ESG) impacts took a real step forward as well. In fact, NASDAQ will now encourage companies on its exchange to report on ESG issues, or explain why they do not.

While these efforts can’t substitute for binding governmental commitments, we are happy to see so many companies and investors stepping up to the plate and believe the private sector will remain a key driver of progress on sustainability issues over the next few years.

Stay Informed in the Months Ahead

Maintaining a well-diversified mix of stocks, bonds, and cash appropriate for your goals and risk tolerance is one of the best ways to mitigate the effects of an uneven economic recovery. We also recommend discussing any changes in your financial situation with your investment advisor.

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A financial services consulting firm recently named Calvert as one of the Top Social Media Leaders in 2012 for using Facebook to share valuable content with investors and advisors. ⁴ Join the dialogue at www.facebook.com/calvert .

We also invite you to visit our website, www.calvert.com , for fund information, portfolio updates, and commentary from Calvert professionals. You can now get the same information on the go with our new iPhone ^® app, available free at iTunes.

As always, we thank you for investing with Calvert.

President and CEO of Calvert Investments, Inc.

October 2012

1. Brookings Institution, Metro Monitor - September 2012, www.brookings.edu/research/
interactives/metromonitor#overall
2. Clear Capital, “June Home Prices Provide Further Evidence of Budding Recovery. Forecast
Indicates Further Increases Through 2012,” July 2012 Market Report, http://clearcapital.com/
company/MarketReport.cfm?month=July&year=2012
3. Paul Davison and Barbara Hansen, “Service Businesses Lead Uneven Jobs Recovery,” USA
Today, July 15, 2012, www.usatoday.com/money/economy/story/2012-07-15/jobs-recov-
ered/56242656/1
4. kasina ranked Calvert fifth of 53 asset management and insurance companies for using
Facebook to tell a compelling brand story and share valuable content with investors and advi-
sors. The ranking was based on kasina’s three-tier methodology that ranked firms based on
content, branding, interactivity, and usability on each social platform. For more information,
visit www.kasina.com.

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Landmark Rules on Conflict Minerals

After a complicated and controversial two-year rulemaking process, the Securities and Exchange Commission (SEC) released final rules under the Dodd-Frank Wall Street Reform and Consumer Protection Act (Dodd-Frank) Section 1502, which require companies to file and publish reports on their efforts to eliminate the use of “conflict minerals” that have fueled a violent 14-year conflict in the Eastern Democratic Republic of the Congo (DRC).

Calvert played a leading role among investors during the rulemaking process and is pleased with the progress. However, the two- to four-year timeline for compliance and exclusion of mining companies from reporting requirements was disappointing. We will continue to press all companies involved with conflict minerals to step up their efforts to address these issues.

New Standards Will Nearly Double Fuel Efficiency by 2025

Newly finalized corporate average fuel economy (CAFE) standards require cars and light-duty trucks to reach the equivalent of 54.5 mpg by 2025. Experts estimate the new standards will reduce oil consumption by 12 billion barrels while saving consumers $1.7 trillion. The major U.S. automakers supported the new standards and have already begun incorporating them into future vehicles.

The higher standards should boost both the competitiveness of American automakers and U.S. policy efforts to mitigate climate change, since more than 90% of a vehicle’s contribution to climate change through greenhouse gas emissions (GHGs) is related to fuel consumption. ¹ Calvert has long advocated for higher standards in public testimony to Environmental Protection Agency, meetings with Obama administration officials, dialogues with the auto industry, and the media and will continue to do so.

Helping Companies Combat Climate Change Risks

Economic and insured losses from natural catastrophes set new records last year, driving home the very real physical climate risks for investors. Extreme weather events accounted for 90% of the disasters and eight of the 10 most costly events, causing overall losses of more than $148 billion.

To that end, Calvert co-authored two reports to help companies evaluate their exposure to climate risks and protect shareholder value. Oxfam America and Ceres joined us to create

Physical Risks from Climate Change: A guide for companies and investors on disclosure and management of climate impacts .

We also developed Value Chain Climate Resilience: A guide to managing climate impacts in companies and communities , a first-of-its-kind guide to help businesses assess and prepare for the risks and opportunities posed by climate change, with other leading companies from the Partnership for Resilience and Environmental Preparedness (PREP).

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Exercising Our Say on Pay

Calvert fought for the new “say on pay” rule, which requires publicly traded U.S. companies to allow shareholders to vote on the compensation awarded to senior management the previous year. We built on that success this year by creating a strict set of voting guidelines on executive compensation for the proxy season. We also shared our reasons for voting against the proposals to management at key companies.

Overall, Calvert votes against compensation packages that do not align management and shareholder interests and do not incentivize the company’s long-term performance. In May, this meant voting against company compensation packages 34% of the time. This includes Gilead Sciences, Safeway, and Plains Exploration and Production, all of which rewarded CEOs with high pay in 2011 despite lackluster shareholder returns.

Even before the May announcement of a massive trading loss, we rejected the $23 million pay package for JPMorgan Chase Chairman and CEO Jamie Dimon due to its discretionary nature and inherent lack of accountability to long-term financial goals. The weaknesses in risk oversight evident by the company’s failed hedging strategy only reinforced our decision.

Other Shareholder Advocacy Efforts

This spring, Calvert filed 24 shareholder resolutions to keep the sustainability issues with a real impact on profits, people, and the planet front and center. Six were voted upon by shareholders, including resolutions on: -- Board diversity at Urban Outfitters, which received 39% support (up from 22% last year). Despite owning popular women’s retail brands Anthropologie and Free People, the company does not have any women or minorities on its board of directors.

-- Disclosure of water scarcity and pollution risks at Fossil, which received 31% support. As an apparel company, Fossil relies on water- and energy-intensive processes for producing cotton, leather, and other products. These processes can also pollute water supplies, especially in developing countries.

-- Sustainability reports at Gentex, which received 32% support. This supplier for the automotive, aerospace, and commercial fire protection industries currently provides no information on its efforts to manage resource efficiency and environmental impacts—despite increasing demands from its key clients for sustainability data.

Promoting Privacy on the Internet

In 2006, Calvert joined other investors, human rights advocacy organizations, academic experts and companies to develop the multi-stakeholder Global Network Initiative (GNI) to help information and communication technology (ICT) companies address freedom of expression and privacy issues. In early 2012, GNI’s path to becoming a fully global standard leapt forward as the first three GNI companies—Yahoo, Google, and Microsoft— completed the second phase of the assessment process.

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In addition, Facebook became a GNI observer in May, opening the door to its participation in GNI policy and advocacy discussions for one year as it evaluates the potential commitment of implementing the GNI principles. We welcome the opportunity to work with Facebook, particularly in protecting against privacy violations that can endanger members using it as an instrument of freedom of expression and political dissent across the world.

Community Investments

Many of our Funds participate in Calvert’s High Social Impact Investing program, which is administered through the Calvert Foundation. This community investment program may allocate a small percentage of Fund assets at below-market interest rates to investments that provide economic opportunity for struggling populations. ² One such investment through the Foundation is FMM Popayan, which offers financial services to more than 400,000 clients in some of the poorest areas of Columbia. Women comprise 66% of their client base.

We also supported the Triodos Sustainable Trade Fund, which seeks to improve the economic position of farmers in emerging markets and stimulate development in the sustainable agriculture sector. It does this by providing pre-export value chain financing to farmer cooperatives—with a specific focus on fair trade and organic farming.

Special Equities

A modest but important portion of certain funds is allocated to small private companies developing products or services that address important sustainability or environmental issues facing our society.

One example of a recent investment is Ivy Capital/All Life Insurance Company of South Africa, which insures people living with HIV. ³ The company uses online support systems to help clients comply with their medical requirements in order to live a longer life. This provides enormous security to their families and employers and seems to be a rather unique business model in fighting the scourge of AIDS.

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1. ACEEE, Automobiles and the Environment, www.greenercars.org/guide_environment. htm

2. As of September 30, 2012, Calvert Social Investment Foundation (“Calvert Foundation” or “Foundation”) Community Investment Notes represented the following percentages of Fund net assets: Calvert Capital Accumulation Fund 0.51%, Calvert International Equity Fund 1.29%, and Calvert Small Cap Fund 0.48%. The Calvert Foundation is a 501(c)(3) nonprofit organization. The Foundation’s Community Investment Note Program is not a mutual fund and should not be confused with any Calvert Investments-sponsored investment product.

3. As of September 30, 2012, Ivy Capital All Life Insurance represented 0.02% of Calvert Equity Portfolio. Holdings are subject to change.

As of September 30, 2012, the following companies represented the following percentages of net assets in Calvert Social Index Fund: Gilead Sciences 0.59%, Safeway 0.04%, Plains Exploration and Production 0.06%, JPMorgan Chase 1.80%, Urban Outfitters 0%, Fossil 0.05%, Gentex 0.03%, Yahoo 0.19%, Google 2.33%, Microsoft 2.62% and Facebook 0%. Holdings are subject to change.

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Investment Performance

Calvert Social Index Fund Class A shares (at NAV) returned 29.36% for the 12-month period ended September 30, 2012. The Fund underperformed its benchmark, the Calvert Social Index (CSI), which posted a return of 30.36% for the period. The difference was primarily the result of fund expenses, which an index does not incur.

Investment Climate

Gradually improving U.S. economic data, healthy corporate earnings, and some policy steps toward mitigation of the sovereign debt crisis in Europe provided support for equities worldwide during the fiscal year. Global inflation remained tame, and aggressive, accommodative monetary policy by central banks around the globe helped equity markets rally hard off their lows posted last fall, albeit on low volume.

For the trailing 12-month period, the Standard and Poor’s (S&P) 500, Russell 1000, and Russell 2000 Indices returned 30.20%, 30.06%, and 31.91% respectively. Negative headwinds from the eurozone and an economic slowdown in China led to a divergence in performance by international stocks with the MSCI EAFE and MSCI Emerging Markets Indices returning 14.33% and 17.33%, respectively.

*Investment performance/return at NAV does not reflect the deduction of the Fund’s maximum 4.75% front-end sales charge or any deferred sales charge.

** See note on page 9 regarding Class Y shares.

Value stocks slightly outperformed growth stocks during the fiscal year, and within the Russell 1000 Index, Consumer Discretionary, Telecommunication Services, and Financials were the top-performing sectors, while the Utilities, Consumer Staples, and Energy sectors lagged.

The U.S. economy continued to proceed on the path of gradual recovery. The U.S. corporate sector remained strong and provided some upside surprises for investors. The top-line numbers driven by the economic recovery were encouraging. Improvement in employment numbers were also a welcome, positive sign, though the decline in the unemployment rate was, unfortunately, primarily due to a drop in the labor force participation rate.

U.S. inflation remained low, bank lending continued to improve, and a weak U.S. dollar, thanks to the especially accommodative monetary policy in the U.S., helped U.S. exports and supported the rebuilding of the domestic manufacturing and industrial base. Vehicle sales and production also looked encouraging.

The manufacturing sector continued to provide a boost to employment and the U.S. economy, but accelerating recession in Europe and the overall slowdown in the global economy will de-emphasize the contribution of exports to U.S. GDP. Therefore, the continued recovery of the U.S. consumer will be important for a self-sustained U.S. economic recovery. More recently, the service sector has been showing signs of improvement. Consumer spending patterns remained promising and consumer confidence, helped by strength in the housing market, a falling unemployment rate, and improving equity markets, has also shown signs of improvement.

* Calvert Social Index Fund first offered Class Y shares on July 13, 2012. Performance prior to that date reflects the performance of Class A shares at net asset value (NAV). Actual Class Y share performance would have been different.

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G rowth of $10,000

The graph below shows the value of a hypothetical $10,000 investment in the Fund over the past 10 fiscal year periods. The results shown are for Classes A, B, C and I shares and reflect the deduction of the maximum front-end sales charge of 4.75%, or deferred sales charge, as applicable and assume the reinvestment of dividends. The result is compared with benchmarks that include a broad based market index and a Lipper peer group average. Market indexes are unmanaged and their results do not reflect the effect of expenses or sales charges. The Lipper average reflects the deduction of the category’s average front-end sales charge. The value of an investment in a different share class would be different.

All performance data shown, including the graph above and the adjacent table, represents past performance, does not guarantee future results, assumes reinvestment of dividends and distributions and does not reflect the deduction of taxes that a shareholder would pay on the Fund’s distributions or the redemption of the Fund shares. All performance data reflects fee waivers and/or expense limitations, if any are in effect; in their absence performance would be lower. See Note B in Notes to Financial Statements. The investment return and principal value of an investment will fluctuate so that an investor’s shares, when redeemed, may be worth more or less than their original cost. Current performance may be lower or higher than the performance data quoted; for current performance data visit www.calvert.com. The gross expense ratio from the current prospectus for Class A shares is 0.97%. This number may differ from the expense ratio shown elsewhere in this report because it is based on a different time period and, if applicable, does not include fee or expense waivers. Performance data quoted already reflects the deduction of the Fund’s operating expenses.

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The U.S. housing market continued its bottoming-out process into early 2012 and showed significant improvement in the second and third quarters of 2012. With mortgage rates at record lows, housing activity picked up throughout the fiscal year. Housing can provide a positive surprise for the U.S. economy and equity markets through the powerful multiplier effect it can have on the consumer and, therefore, the economy.

The stance of the Federal Reserve (Fed) remained that it was ready to act should economic conditions in the U.S. deteriorate. This so-called “Bernanke put” provided support for U.S. equity markets throughout the fiscal year with investors believing the Fed would provide future liquidity injections in the event the economy faltered. A third round of quantitative easing (QE3) materialized in September with the Federal Reserve’s Open Market Committee (FOMC) announcing an open-ended commitment to purchase $40 billion of mortgage-backed securities each month until substantial improvements in the labor market were seen. This came on top of the FOMC’s announcement earlier in the year to extend “Operation Twist” in an effort to further reduce long-term interest rates.

With global economic challenges keeping inflation in check, policymakers around the globe also continued their efforts toward easing monetary policies. On the flip side, the easing cycle has been inflating the balance sheets of the world’s six biggest central banks, which have more than doubled since 2006.

In the eurozone, the economic picture looked increasingly grim throughout the fiscal year, with peripheral economies firmly in a recessionary spiral and core economies coming under stress. Manufacturing in the region continued to fall deeper into contraction territory while consumer confidence in the eurozone reached its lowest level since early 2009 and the unemployment rate hit a record high.

As we anticipated, markets seemed to overestimate the efficacy of the policy action in Europe and underestimated the possibility of a more severe economic recession in the eurozone.

China cut its economic growth target from 8.0% to 7.5% during the first quarter of 2012, signaling the country’s need to transition from an export-driven to a more sustainable, consumer-driven, economic model. Nevertheless, the Chinese economy continued to decelerate during the fiscal year as foreign direct investment (FDI), one of the major drivers of economic growth in China, continued to decline and China’s HSBC Manufacturing Purchasing Managers Index (PMI) was in contraction territory for 11 consecutive months as exports continued to weaken.

A slower inflation trend allowed the Chinese government to reposition its economic policy from contractionary for most of 2011 to stimulative. A hard landing in China is not out of the question and could significantly impact global growth while making the overall macroeconomic backdrop riskier for the next several quarters.

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Portfolio Strategy

Calvert Social Index Fund is managed using a passive investment strategy, with the objective of matching the day-today performance of the CSI as closely as possible. We do this by buying all, or virtually all, of the stocks in the CSI and holding them in the same proportion. Variation in the Fund’s total return generally reflects the variation in the total return of the CSI.

Information Technology, Financials, Health Care, and Consumer Discretionary are the four largest sectors in the CSI and accounted for almost 70% of its value, on average, over the reporting period. Therefore, performance in these sectors has a significant impact on overall performance. Financials was the worst-performing sector, returning -17%.

Information Technology, Health Care, and Consumer Discretionary all posted positive returns.

Utilities was the best-performing sector in the CSI, gaining 17%. Consumer Staples also produced good returns with a gain of more than 10%. The decline of almost 17% in the Financials sector was the biggest drag on performance, and the Industrials sector declined by almost 9%.

Outlook

Despite healthy corporate earnings and continued gradual economic recovery in the United States, looming developments in the political landscape could have a negative impact on U.S. GDP growth and are likely to be a source of volatility for the markets in the coming months. However, once we have more visibility into the “fiscal cliff” scenarios, market participants may feel better about investing in risky assets for the long term.

We see continued challenges in the global macro backdrop leaving equity markets vulnerable to negative news, especially given that most of the positive catalysts are out and the markets had a healthy run through September 30, 2012. Any negative news from Europe or China or a soft earnings season in the United States will likely trigger a risk aversion trade.

October 2012

SHAREHOLDER EXPENSE EXAMPLE

As a shareholder of the Fund, you incur two types of costs: (1) transaction costs, including sales charges and redemption fees; and (2) ongoing costs, including management fees; distribution (12b-1) fees; and other Fund expenses. This Example is intended to help you understand your ongoing costs (in dollars) of investing in the Fund and to compare these costs with the ongoing costs of investing in other mutual funds.

This Example is based on an investment of $1,000 invested at the beginning of the period and held for the entire period (April 1, 2012 to September 30, 2012).

Actual Expenses

The first line of the table below provides information about actual account values and actual expenses. You may use the information in this line, together with the amount you invested, to estimate the expenses that you paid over the period. Simply divide your account value by $1,000 (for example, an $8,600 account value divided by $1,000 = 8.6), then multiply the result by the number in the first line under the heading entitled “Expenses Paid During Period” to estimate the expenses you paid on your account during this period.

The Fund charges an annual low balance account fee of $15 to those shareholders whose regular account balance is less than $5,000 ($1,000 for IRA accounts). If the low balance fee applies to your account, you should subtract the fee from the ending account value in the chart below.

Hypothetical Example for Comparison Purposes

The second line of the table below provides information about hypothetical account values and hypothetical expenses based on the Fund’s actual expense ratio and an assumed rate of return of 5% per year before expenses, which is not the Fund’s actual return. The hypothetical account values and expenses may not be used to estimate the actual ending account balance or expenses you paid for the period. You may use this information to compare the ongoing costs of investing in the Fund and other funds. To do so, compare the 5% hypothetical example with the 5% hypothetical examples that appear in the shareholder reports of the other funds.

Please note that the expenses shown in the table are meant to highlight your ongoing costs only and do not reflect any transactional costs, such as sales charges (loads) or redemption fees. Therefore, the second line of the table is useful in comparing ongoing costs only, and will not help you determine the relative total costs of owning different funds. In addition, if these transactional costs were included, your costs would have been higher.

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* Expenses are equal to the Fund’s annualized expense ratio of 0.75%, 1.75%, 1.73%, 0.21%, and 0.60% for Class A, Class B, Class C, Class I, and Class Y, respectively, multiplied by the average account value over the period, multiplied by 183/366 (to reflect the one-half year period).

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See notes to financial statements.

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(b) This security was valued by the Board of Directors. See Note A.

^ Futures collateralized by $650,000 par value of U.S. Treasury Bills.

* Non-income producing security.

Abbreviations:

plc: Public Limited Company

See notes to financial statements.

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