A woman was threatened with a knife and raped as she crossed a sports field in an Oxfordshire village.
Thames Valley police said the incident happened at about 2140 BST on 15 August, off the High Street in Eynsham.
Officers said the man told the 50-year-old woman that he had a knife and forced her to perform a sex act on him.
The women pushed the man away and escaped. The attacker is described as white, about 6ft tall and aged in his 20s.
He was wearing white jeans, a white puffer-style jacket and trainers.
Det Sgt Rob Hughes appealed for witnesses to come forward.
He added: "The neighbourhood police team is aware of the incident and patrols will be stepped up in the village to reassure the local community."