An elderly woman on a stroll near Kettering lake saw a man indecently expose himself.
Police are appealing for witnesses after the incident at the lake near Kettering Conference Centre in Thurston Drive between 4pm and 4.30pm on Tuesday, June 25.
The offender passed an elderly woman on the footpath which surrounds the lake.
Later along the path she saw the man again, realising he had his trousers open and was exposing himself.
A police spokesman said: "He is described as a white man wearing a black blazer and dark trousers."
Anyone with information should call Northamptonshire Police on 101 or Crimestoppers, anonymously, on 0800 555111.