Conservationists are asking for help to carry out a survey of water vole populations.
People in Hartlepool have been urged to report sightings of the increasingly rare animal by filling in database forms produced by Hartlepool Borough Council's ecologist, Dan McAndrew.
The town has several colonies, most notably in the grounds of Hartlepool Power Station and Huntsman Tioxide, as well as at Seaton Common, Soverign park in Brenda Road and along Greatham and Dalton becks.
However, it is believed that others are waiting to be discovered.
This view is supported by two independent reports of water voles living in a stream in Hart Village.
After an investigation, evidence was found to indicate that the animals were present.
Countryside warden Robert Smith said yesterday: "Water voles are often mistaken for rats and this has resulted in much unintentional persecution.
"The main differences to look out for are the shorter furry tail and the blunt nose and small ears.
"They are likely to occur in any beck or ditch, even in a garden pond that has a bank to burrow into."
Survey forms are available from (01429) 523431. Water vole sightings can be reported by calling (01429) 853325.
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