Raising new questions about the environmental risks of some widely used farm chemicals, scientists are reporting today the first evidence linking agricultural runoff to grotesque hind-limb deformities in frogs.

Researchers said frogs appear to be made more vulnerable to a common parasite when exposed to the pesticides atrazine and malathion. The parasite, a burrowing trematode worm, tends to infect the hindquarters of developing tadpoles.

Atrazine is part of a family of chemicals that rank among the world's most widely used weed killers. Malathion is commonly applied to control mosquitoes and other insects, and pharmaceutical grades are approved for killing head lice. Both products are controversial but considered safe for commercial use in the United States.

Now, effects of these and other chemicals on the environment are coming under new scrutiny. Research is driven partly by keen public and scientific interest in the declining health of amphibian populations, often portrayed as a sentinel for environmental decline and a possible early warning of health problems affecting humans.

At last count, wild frogs with missing or extra hind limbs have been observed in at least 43 states and five Canadian provinces. Earlier studies clearly implicated the trematode parasite but left open the question of what might be causing the apparent increase in the problem.

The latest study, by ecologist Joseph Kiesecker at Pennsylvania State University and edited by UC Berkeley amphibian specialist David Wake, tries to fit in the key remaining puzzle piece. The study appears in the early edition of this week's Proceedings of the National Academy of Sciences.

Kiesecker said his observations of the common wood frog Rana sylvatica in the wild, followed by controlled studies in his laboratory, produced "compelling" evidence that pesticides can weaken the immune system of exposed amphibians -- even at very low concentrations -- making the frogs more vulnerable to parasites.

The field studies showed "considerably higher rates of limb deformities where there was pesticide exposure," Kiesecker said in an interview. "Then the lab experiments helped support the mechanism for what we saw in the field."

He also looked at another pesticide, a synthetic chemical called esfenvalerate, but did not find the same links to growth anomalies as seen with malathion and atrazine.

For the latter two chemicals, significant effects were seen even at concentrations considered safe for drinking water by the Environmental Protection Agency.

Even these very low levels of exposure could produce "dramatic effects on the immune response" of the animals. And that, in turn, led to significantly more growth defects.

Kiesecker stopped short of endorsing any effort to further restrict use of atrazine and malathion. But he said his results underscored the importance of studying toxic chemical effects in a context approaching the complexity found in natural ecosystems.

In this case, he explained, the two farm chemicals "disturbed host-pathogen interactions" with sometimes devastating effects. But all that would be missed in traditional studies examining only the chemicals and the frogs in isolation.

Some other scientists, backed by the farm-chemical industry, challenged Kiesecker's results. Although they said the new study was intriguing, they suggested the details couldn't be trusted until corroborated independently.