By Kirsten Grind 

Investors pulled $18.5 million from Bill Gross's new fund at Janus Capital Group Inc. in February, the first monthly withdrawal since he moved to the firm in September, according to data from research firm Morningstar Inc.

After a fast start, inflows into the Janus Global Unconstrained Bond fund have been declining since December. The slowdown has led some analysts and industry observers to question Mr. Gross's ability to capture some of the $190 billion that has left his former home, Pacific Investment Management Co., since his departure last fall.

Assets in Mr. Gross's fund at Janus sank to $1.45 billion at the end of February from about $1.5 billion a month earlier, according to Morningstar. Overall, Janus's mutual funds saw net inflows of $449.5 million in February. Asset totals also take into account market appreciation and other factors.

A Janus spokeswoman declined to comment.

Mr. Gross, who previously ran Pimco's flagship Total Return fund, the world's biggest bond fund by assets, could be hamstrung by his fund's new strategy, which is unfamiliar to many investors, said Jeff Tjornehoj, head of Americas research at fund tracker Lipper. An unconstrained bond fund is able to invest in a wider range of securities than a total return fund.

"It's a very different portfolio than what he's best known for at Pimco, " Mr. Tjornehoj said. "It's going to have some limited appeal."

Mr. Gross abruptly left Pimco, where he was chief investment officer and co-founder, after rising tensions with executives at the firm and a period of spotty performance. When he left, the big question on the minds of investors and analysts was how much money he would be able to attract at a new firm.

Initially, it appeared Mr. Gross was doing fairly well, with his new fund jumping in asset size from about $12 million when he took over in September to more than $1 billion by November. But after a report in The Wall Street Journal, Janus confirmed in January that Mr. Gross had invested "more than $700 million" in the fund himself.

Janus, with a total of $183 billion in assets under management, had suffered outflows from its mutual funds for years before Mr. Gross's arrival, and some analysts and investors hoped he would help buoy the company. The company's stock, which saw its biggest gain in its 45-day history on the day Mr. Gross arrived, has gained about 13% since early October. Monday afternoon, Janus's stock was up 1.6%, to $16.56.

Citigroup analyst William Katz said in a research note in early February that Janus investors were overestimating the potential for inflows related to Mr. Gross and affirmed his "sell" rating on the firm. The note came after Morningstar reported January inflows of $86 million into Mr. Gross's fund, the smallest amount since he arrived at the firm. "Volumes related to Bill Gross remain underwhelming," Mr. Katz wrote.

Since he began trading in his fund Oct. 6, Mr. Gross has lost 1.2% through March 6, lagging behind 83% of similar funds, according to Morningstar.

Meanwhile, investor outflows from Pimco's Total Return fund are slowing. The company said last week that the fund saw $8.6 billion in outflows, the lowest monthly amount since Mr. Gross's departure. The fund had $124.7 billion in assets at the end of February.

Write to Kirsten Grind at kirsten.grind@wsj.com

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