Two girls were touched inappropriately as they waited in a bus queue in Kettering.
The incident happened on Saturday, June 13, between 3pm and 4pm by the bus stop in Newland Street.
The two girls, both aged 16, were grabbed from behind by a man.
One of them screamed and he left the scene.
The offender is described as white, aged 40 - 50 with short, messy hair and a stubbly complexion.
He wore a light-green fleece top and black jeans and trainers. He had blood on his hands and cuts on his face with dried blood above his eyes.
There were lots of people around at the time and police are appealing for witnesses to come forward, including the driver of the bus, which was the X4 Corby bus.
Witnesses, or anyone who knows anything about this incident, can call Northamptonshire Police on 101 or Crimestoppers anonymously on 0800 555111 or use the anonymous online form at www.crimestoppers-uk.org.