A Brighton woman was sexually assaulted as she travelled on a night bus towards Falmer.
The victim, 24, boarded the number 25 night bus in Churchill Square in the early hours of the morning on Wednesday 18 August.
Police want to trace a man who boarded the bus in the Old Steine and who they believe may have seen the assault.
Detective Constable Mors O’Connor said: “This is a serious and distressing crime.
“I’m keen to speak to a man who may not have realised any crime was being committed, but may still hold information pertinent to this investigation.
“The witness is believed to have got on the bus at the Old Steine and spoken to the suspect, who initially sat next to him.”
The incident happened at about 2.15am last Wednesday.
The witness got off the bus before the victim alerted the bus driver in Coldean Lane.
The driver called the police.
An 18-year-old man from Brighton was arrested in Ditchling Road shortly after the incident on suspicion of sexual assault by touching.
He was released on bail until Monday 4 October.