McLEAN, Va. -- Some species of male fish in the Potomac River and its tributaries are developing female sexual traits at a frequency higher than scientists have seen before, raising concerns about pollutants in a waterway that provides drinking water for millions of people.

The so-called "intersex fish," which produce immature eggs in their testes, were discovered in the Potomac rivershed in 2003 and have also been found in other parts of the country.

But the frequency that the U.S. Geological Surveys found last year is much higher than what has been found elsewhere, said fish pathologist Vicki Blazer.

In some Potomac tributaries, nearly all of the male smallmouth bass caught in last year's survey were the abnormal fish. In the Potomac itself, seven of 13 largemouth bass exhibited female characteristics, including three that were producing eggs.

Although the frequency discovered was surprisingly high, Blazer cautioned that the sample size was relatively small, with about 10 male and 10 female fish taken from each of eight locations in Maryland, Virginia and the District of Columbia.

Researchers were reluctant to remove large numbers of bass from the rivers because of conservation concerns, she said.

Female fish caught in the survey did not develop any unusual sex traits, though fish of both sexes exhibited lesions and other pollution-related problems, said Blazer, who coordinated the survey.

Smallmouth bass appear to be more susceptible to intersex development than largemouth bass, Blazer said.

Blazer said researchers are still waiting on data that would help them determine the water quality at the time the fish were caught, but preliminary data taken from the Potomac found a variety of chemical pollutants.

It is not exactly clear what is causing the changes, though it is likely a combination of pollutants, scientists say.

Certain chemicals and pesticides are believed to stimulate estrogen production. Also, estrogen from birth control pills and human waste can make its way from sewage treatment plants to the waterways.

The Environmental Protection Agency has been studying the issue of so-called "endocrine disruptors" since 1996, but currently does not issue guidelines to water treatment plants for allowable levels of estrogenic compounds.

Jeanne Bailey, a spokeswoman for Fairfax Water, said the findings are a concern.