A man exposed himself in front of a teenage girl and performed a sexual act.
It happened on the top deck of the number 36 First bus travelling from Bolton towards Swinton at about 5.15pm on 31 December 2012.
The young girl, aged 15, was sat on the front row of the top deck when the offender got onto the bus on Thynne Street. He sat opposite the teenager.
He then exposed himself and performed a sexual act in front of her.
Distressed, the girl got up when the bus stopped on Plodder Lane and walked downstairs to get away, but was followed by the man. However, the man got off the bus at this stop while the victim went to the back of the bus to escape him.
Police have now released CCTV stills of the offender and are appealing to anyone who recognises him to come forward.
He is described as black, in his mid 40s, with short, dark curly hair and stubble. He was said to be of an average to stocky build, about 5ft 6in and was wearing a black coat and black trousers.
Inspector Charlotte Cadden said: "Any parents reading this will understandably be anxious and that is the reason we have issued this appeal and released CCTV stills of the suspected offender.
"We want anyone using public transport to be vigilant and if you think you recognise this man then please either call police immediately or inform the driver straightaway and move to a safe part of the bus, tram or train.
"We do not want people to be unduly alarmed or avoid using public transport but it is important that people are safe and be on the look out for this individual.
"Anyone with information should call police on 0161 856 1576 or the independent charity Crimestoppers, anonymously, on 0800 555 111."