A man exposed himself and touched a woman's bottom while she walked with her friend in Northampton.
A spokesman for Northamptonshire Police said: "The sexual assault happened in Harborough Road, Kingsthorpe, between 9.30pm and 11.30pm on Thursday, January 25.
The friends were walking near the BP Garage when a man started to follow them.
He approached the victim and touched her bottom. He was exposing himself while the incident took place.
The alleged offender was white, in his late teens, of slim build and about 5ft 5in. He wore a dark hooded top and tracksuit bottoms.
Officers would like to speak to anyone who may have witnessed this incident or who may have any information regarding it.
Witnesses or anyone with information can call police on 101 or Crimestoppers anonymously on 0800 555 111.