Washington

Rita R. Colwell, the director of the National Science Foundation, has resigned, according to a Congressional aide, and Arden L. Bement Jr., the director of the National Institute of Standards and Technology, will replace her, effective February 21.

Officials at the National Science Foundation and the National Institute of Standards and Technology could not be reached for comment at the close of business Tuesday.

Ms. Colwell is scheduled to testify today before the Committee on Science of the U.S. House of Representatives in a hearing on President Bush's budget requests for federal science agencies in the 2005 fiscal year. It is expected that she will speak about her departure at the hearing.

Last week, Ms. Colwell held a briefing on the $5.745-billion budget request for the science foundation. At the time, she declined to comment on rumors that her departure was imminent.

Ms. Colwell, the former president of the University of Maryland Biotechnology Institute, took office in 1998 under President Bill Clinton. She remains a professor of biotechnology but is technically on leave.

Some science advocates said the timing of the departure was unusual, given that Ms. Colwell's term was set to expire in August. It is not known what Ms. Colwell plans to do when she leaves the NSF.

Ms. Colwell is credited with increasing the science foundation's budget from $3.4-billion in the 1998 fiscal year to an estimated $5.58-billion this year. The agency is one of the largest sources of grants for university research.

The appointment of Mr. Bement, a former member of the National Science Board, is expected to be temporary.