Officers are appealing for witnesses after an incident in Northampton.
Just before 7am on Friday 10 March, a woman was walking under the bridge at St James, near to Mill Road. A man ran towards her, indecently exposed himself and touched himself inappropriately. The woman screamed and he ran off.
The offender is described as a white man in his late 30s, wearing jeans, trainers, a hooded top, a gilet and a cap.
Witnesses can contact police on 101 or Crimestoppers anonymously on 0800 111 555.