A 34-year-old woman was raped in Glasgow city centre in the early hours of Friday (14 August).
The woman was in Cadogan Street at about 05:00 when she was attacked by a man.
Police said the victim, her friend and the man then boarded a number 61 bus on Hope Street at about 06:18.
The man was described as in his 20s, sallow-skinned and wearing dark clothing.
Det Insp Diane Caldwell said: "We are looking to speak to anyone that was travelling on the number 61 bus around this time who may have witnessed a disturbance involving two women and a man.
"Officers are carrying out inquiries locally, examining CCTV and patrols have been stepped up around the city centre to provide reassurance to members of the public."
Anyone with information about this incident is asked to contact police on 101 or Crimestoppers on 0800 555 111 where all calls will be kept anonymous.