Environmental experts are investigating after malformed frogs were found in a stream in Peterborough.
The deformity might have been caused by a parasite. Photo courtesy Natural England.
Natural England said it did not know at this stage what caused the common frogs to produce five legs but parasitic attack or pollution are possibilities.
The amphibians were found by a member of the public at Gunthorpe near the Cuckoo's Hollow beauty spot.
It is thought to be the first such incident in the UK although there have been reported sightings in the US.
Apart from the apparent deformity the frogs seem to be in good condition and are spawning.
A spokeswoman for the agency said: "There has been research in the United States which suggests that the deformity might be caused by a parasite which attacks frogspawn and disrupts growth patterns."
She added: "The frogs must be one or two years old, so whatever happened didn't happen this year."
Because of their porous skin amphibians are also susceptible to chemical contamination.
The Environment Agency has had reports from people of pollution at the site.
In 2004 a company was prosecuted for allowing detergent to run into a surface water drain polluting the site.
Several hundred dead fish were found around the Cuckoo's Hollow area.