Police have released CCTV images of a man they want to trace after a schoolgirl was bitten on the face as she got off a train.
The 14-year-old was involved in an altercation with her attacker during a train journey and was assaulted by him as she left the train at Aberdare.
She suffered a bite mark on her forehead and bruising to her face.
British Transport Police said it was a "frightening experience".
Theyhave been unable to locate the man in the picture and are appealing for help.
PC Nigel Maggs, the investigating officer, said: "This was a nasty incident which left the victim injured and in some distress.
"It was a frightening experience for the girl, who did not know her attacker.
"Behaviour such as this is completely unacceptable and will not be tolerated on the rail network.
"It is essential that we identify the person responsible for this attack, and I urge anyone with any information to contact police."
The incident happened at around 16:40 BST on Saturday, 2 June.
Police said they believed the man in the picture had information which could help the investigation.
He was wearing a black Nike T-shirt and black flat cap, and had a distinctive Mohican-style haircut.
"The CCTV footage is clear and I'm sure someone will recognise the man pictured," added PC Maggs.
It is believed he got on the train at Cwmbach. The teenager had boarded the train at Pontypridd.
British Transport Police have asked anyone who knows the man's identity, or his current whereabouts to contact them on 0800 405040, or Crimestoppers on 0800 555 111. Callers are asked to quote incident reference B4/WCA of 09/07/2012.