Police are looking for a man who exposed himself to a woman on August 8, on Old Coach Road, off Burton Road, Lincoln.
A woman was walking her dog at around 9.30am when a man, naked from the waist down, approached her masturbating.
Police say there was no physical contact and no communication. The man then left the scene in a white van.
The offender is described as white and in his early 20s. He was 6ft tall and slim and was wearing a dark colored hooded top, a red T-shirt and khaki trousers.
Anyone with information can call Lincolnshire Police on 0300 111 0300.