A man who complained about a loud phone conversation on a train bit a fellow passenger. The attacker had been complaining that the victim's friend was talking loudly on a Southern Railway service from London Bridge to Peckham Rye, British Transport Police said.
An argument ensued during which the attacker leaned over and began biting the passenger's ear.
A BTP spokesman said the attacker "eventually stopped biting the victim's ear" and returned to his seat.
The victim. who was 24, suffered a deep cut to his ear during incident on 6 October and was taken to hospital for treatment.
Officers released a CCTV image of a man they believe may have information which could help the investigation.
Anyone who recognises the man pictured is asked to call BTP on 0800 40 50 40 or text 61016, quoting reference number 376 - 08/10/2017.