Experts are investigating after frogs with five legs were found in a river.Environmental agency Natural England said specialists were puzzled by the deformity.
A spokeswoman said it was thought to be the first time such frogs had been spotted in the UK.
Five-legged frogs have previously been seen in the US.
The UK batch were discovered in Peterborough, Cambridgeshire, by a member of the public.
"It would be interesting to hear from anyone who has seen them," said the spokeswoman.
"We are not sure what the reason is. A parasite; pollution - they are possibilities. But we've reached no conclusions."
She said research in the US suggested 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."
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