A man indecently exposed himself to a 17-year-old girl as she was walking along the underpass from Norton Road to Abbey Street in Daventry.
The incident happened on Tuesday, October 13 at about 1am.
The man turned towards the girl and dropped his trousers as she passed, exposing himself.
He is described as white, in his 20s and about 5ft 7in tall. He was bald and possibly had three stars tattooed on his head. He wore blue tracksuit top and matching bottoms.
Witnesses, or anyone who knows anything about this incident, can call Northamptonshire Police on 101.
Alternatively, they can call the independent charity Crimestoppers anonymously on 0800 555 111 or use the anonymous online form at www.crimestoppers-uk.org.