Atrazine, one of the most commonly used and controversial weedkillers, can turn male frogs into females, according to researchers.
The experiment is the first to show such complete effects of atrazine, which had been known to disrupt hormones and which is one of the chief suspects in the decline of amphibians such as frogs around the world.
"Atrazine-exposed males were both demasculinised (chemically castrated) and completely feminised as adults," Tyrone Hayes of the University of California Berkeley and colleagues wrote in the Proceedings of the National Academy of Sciences.
The chemical had been shown to disrupt development and make frogs develop both male and female features - termed hermaphroditism. This study of 40 male frogs shows the process can go even further, said Dr Hayes.
"Before, we knew we got fewer males than we should have, and we got hermaphrodites. Now, we have clearly shown that many of these animals are sex-reversed males," Prof. Hayes said in a statement.
"Atrazine has caused a hormonal imbalance that has made them develop into the wrong sex, in terms of their genetic constitution."
Whether the effects translate to humans is far from clear. Frogs have thin skin that can absorb chemicals easily and they literally bathe in the polluted water.
The EU banned atrazine in 2004. The finding may add pressure to the US to more closely regulate the chemical, used widely in agriculture.
"Approximately 36,287 tonnes are applied annually in the US alone, and atrazine is the most common pesticide contaminant of ground and surface water," the researchers wrote.
"Atrazine can be transported more than 1,000 kilometres from the point of application via rainfall and, as a result, contaminates otherwise pristine habitats, even in remote areas where it is not used," they added, citing other researchers.
"In fact, more than 227 tonnes of atrazine are precipitated in rainfall each year in the US."
The US Environmental Protection Agency said in October it was reviewing the health impacts of atrazine.
Syngenta AG, one of several companies that makes atrazine, has long defended its safety. The company says it is one of the best-studied herbicides available and pointed to prior safety reviews from the EPA and World Health Organisation, among others.
Prof. Hayes and colleagues studied 40 African clawed frogs, keeping them in water contaminated with 2.5 ppb (parts per billion) of atrazine. The EPA's current drinking water standard is 3 ppb.