NEW YORK (CNN/Money) - Tom Bailey, who founded and grew Janus into one of the nation's top-performing mutual fund families, is stepping down as CEO and president after 33 years at the helm, the company said Wednesday.

The mutual fund group said Bailey, 64, will remain chairman of the growth-oriented Janus funds and will continue working with its trustees to review and monitor fund performance.

"This is a bittersweet moment for me because Janus has been -- and continues to be -- an important part of my life," Bailey said. "I've spent half my life -- and nearly all my professional years -- at Janus. But it's time to shift my focus to my responsibilities as a trustee and away from my day-to-day operational role as CEO."

Janus has been hit hard by the stock market downturn after racking up big gains during the technology-led bull market of the late 1990s.

The group had about $159 billion in assets under management as of May 31, down from more than $300 billion at the stock market's peak in early 2000, as its portfolios lost value amid stock market declines and as frustrated investors cashed out of ailing funds.

Janus, which made its name in the 1990s with big bets on aggressive growth stocks, has been thrashed by bearish performance of two of its top holdings, AOL Time Warner (AOL: down $0.21 to $15.64, Research, Estimates) and Nokia Corp. (NOK: down $0.11 to $12.44, Research, Estimates).

The mutual fund company began heavily selling large chunks of the two companies and added Tyco International to its portfolio, raising its stake in Tyco to about 63.67 million shares at the end of March from 51.27 million at the end of 2001.

Tyco (TYC: down $1.54 to $9.51, Research, Estimates) stock, however, also has fallen sharply in recent months on concerns about a restructuring plan and accounting and profit worries.

Bailey's resignation follows his announcement in October that he would sell his remaining stake in Janus private stock to the company's parent, Stilwell Financial, raising questions about the fund company's future leadership. At the time, he said he had no plans to step down as CEO.

The company's management committee will continue to run the firm's day-to-day operations. Helen Young Hayes, Janus' managing director of investments, and Jim Goff, the fund's director of research, will continue to oversee the research team.

Janus and Stilwell Financial (SV: down $0.42 to $19.14, Research, Estimates) will search for a permanent successor.

Bailey's retirement comes two years after railroad operator Kansas City Southern Industries spun off Janus as a division of Stilwell Financial. Prior to the spinoff, Bailey and other top Janus managers made it clear they wanted Janus to be spun off independently.

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-- Reuters contributed to this story.