A teenager punched a man in the face in Kettering before riding off on his BMX.
Police are investigating after the assault in Grafton Street on Friday night (June 28).
The incident took place between 8.45pm and 9pm when the attacker approached the victim and punched him in the face, causing him to fall backwards onto the pavement.
After he got up the victim chased the offender, who rode off on a mountain bike up Grafton Street and into Oakley Street.
A police spokesman said: "The suspect is described as white, aged 16 to 19-years-old, about 5ft 11in and with a slim build.
"He had short, curly, mousy brown hair and was wearing a tracksuit top and bottoms.
"He was riding a blue BMX-style bike."
Witnesses to the incident, or anyone with information, are asked to call Northamptonshire Police on 101.
Alternatively, they can contact Crimestoppers anonymously on 0800 555111.