The frogs appear normal on the outside, but often have both male and female sex organs, the researchers said, adding that the findings may help explain the amphibian population decline.

The decline worries scientists because amphibians such as frogs respond to environmental dangers before other species.

"Atrazine is the most commonly used herbicide in the U.S. and probably the world," the researchers wrote in their report, published in the Proceedings of the National Academy of Sciences. "It can be present at several parts per million in agricultural runoff and can reach 40 parts per billion in precipitation."

Such a common pollutant would reach many animals as well as humans, so the team, led by Tyrone Hayes, tested its effects on the African clawed frog Xenopus laevis.

They put tadpoles into water laced with levels of atrazine much lower than allowed by the U.S. Environmental Protection Agency, and found that the weedkiller had serious effects on 16 percent of them.

These genetically male frogs had testicles and ovaries, and many had testicles that did not function properly and contained eggs and sperm. Their levels of testosterone were much lower than normal.

The male frogs also had much smaller larynxes than normal, which could affect their ability to croak and attract mates, Hayes' team reported.

The researchers determined that atrazine is an endocrine disruptor - a class of chemicals that disrupts the hormones in the body. Many pesticides fall into this category and it is one of the qualities that can make them dangerous.

"We hypothesize that atrazine ... promotes the conversion of testosterone to estrogen," they wrote.

"This widespread compound and other environmental endocrine disruptors may be a factor in global amphibian declines."

Environmentalists responded with alarm.

"This research is further proof that this pesticide is a major threat to public health and the environment," the Natural Resources Defense Council said in a statement.

"EPA's tap water standard for atrazine is 3 parts per billion - 30 times higher than the level at which these dramatic sexual side effects occurred. At higher levels, the frogs developed additional health problems," it added.

"This rigorous scientific study reinforces what we and other scientists have been saying for years - atrazine is a dangerous pesticide," the council's Jennifer Sass said. "It's no surprise that it's been banned by many European countries."

She said the study had implications for humans, especially children who have not reached puberty.