Police have releasing a CCTV image after a woman said she was "inappropriately touched" on a bus from Watford to St Albans.
The alleged incident took place at about 5.30pm on March 24 when the woman, who is in her 30s, was sat on the Arriva 321 bus.
The woman said someone sat behind her reached around and touched her inappropriately. She moved seats before getting off the bus.
Officers would like to speak with the man pictured as he may have witnessed what happened or have information about the incident.
Anyone with information is asked to contact the Herts Police non-emergency number 101 quoting crime reference number F1/14/986. Alternatively call Crimestoppers, the independent charity, anonymously on 0800 555 111.