These frogs have extra legs, or no legs, fingers protruding from stomachs -- some have an eye in the middle of their back. So far, there have been reports of unusually high numbers of deformed frogs in Minnesota, Vermont, Wisconsin, and Quebec, but the deformities have also been spotted in many other states around the country.

"In the U.S., deformed frogs have been most fully documented in the northeastern and midwestern sections of the country," said Dr. Kathryn Converse, wildlife disease specialist for the National Wildlife Health Center of the U.S. Geological Survey's Biological Resources Division in Madison, Wisconsin. "Now investigation and sampling of frog populations is taking place in other regions, as well."

"At the present time," she explained, "most states don't have an in-hand collection of frogs, so it's impossible to tell how widespread the problem is."

Converse works in a diagnostic laboratory where many deformed frog specimens have been sent for analysis. Herpetologists are trying to build a data-base on deformed frogs, she said, so research can be more systematized.

"We are trying to find what is and is not known about these frogs," she said. "Not only scientists, but, more and more, ordinary people are finding deformed frogs, and we need a systematic sampling to determine the full extent of the problem."

There are a number of studies now underway, said Converse, some linking these deformities to chemicals in the environment.

According to Martin Ouellet of McGill University in Montreal, the deformities are most strongly linked to pesticides. He and four other scientists have been studying deformed and normal frogs found in ponds in the St. Lawrence River Valley for the past four years.

Normally, less than one percent of frogs are deformed, and that's about what Ouellet found in frogs taken from pristine ponds. But in ponds where pesticides are used nearby, as many as 69 percent of the frogs were deformed, he said.

Many scientists believe the problem lies with the new generation of chemicals that mimic growth hormones called retinoids. Retinoids powerfully affect development, and if they are inside a growing animal at the wrong place at the wrong time, they can cause deformities.

Recent laboratory experiments have determined that a pesticide can mimic a retinoid and, conceivably, cause defects in frog development, said David Gardner, a molecular biologist from the University of California at Irvine.

"We should start screening chemicals, and we should start with pesticides to see if they mimic naturally-occurring retinoids in the body," Gardner said.

Other scientists link the deformities to parasites, but many are sceptical of his theory.