A popular 46-year-old herbicide used throughout the U.S. Midwest on export crops appears to have the unintended effect of turning male leopard frogs into females, according to research released Wednesday.

"Atrazine is potentially destroying biodiversity. In my opinion, this is an unacceptable risk," said study author Tyrone Hayes, an associate professor of integrative biology at UC Berkeley.

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Testing their findings in the lab, scientists from the University of California, Berkeley, have found that male leopard frogs exposed to even tiny amounts of the weed killer, atrazine, grew egg cells in their testes, essentially turning them into hermaphrodites.

The findings, published in the Oct.31 issue of the journal Nature, showed that these sexual abnormalities turned up when concentrations of atrazine was as low as 0.1 parts per billion (ppb), one-thirtieth the amount of the chemical allowed in drinking water by the Environmental Protection Agency.

These findings, combined with earlier studies showing the negative effect atrazine has on several species of frog, suggest that the weed killer could be a factor in the decline of frogs and other amphibians in the United States.

"These studies clearly indicate that atrazine is detrimental to amphibians," Mr. Hayes said.

"Testes in these male frogs are obviously not functioning normally, because if they were, egg cells would not be able to grow in them," he said. "Some testes are so invaded by ovary cells it looks like they are converted, and technically they could be considered ovaries."

Mr. Hayes reported earlier this year that a common laboratory frog, the African clawed frog, becomes demasculanized when raised at exposures of 0.1 ppb of atrazine. The EPA has set 3 ppb as the allowable limit of atrazine, the proposed chronic exposure limit for aquatic life if 12 ppb.

"The current data raise concern about the effects of atrazine in general and the potential role of atrazine and other endocrine-disrupting pesticides in amphibian declines."

Atrazine has been used on crops since 1956, the scientists say, and is currently the most widely used herbicide in the United States. It is regularly used on corn and soybeans, including those crops destined for export.