A youth was threatened with a knife in Northampton town centre before having his bike stolen.
Northamptonshire Police detectives want to talk to anyone who witnessed the robbery at the junction of The Drapery and Gold Street on Monday evening.
At around 9.15pm, a 15-year-old boy was sitting on his blue and green Carrera mountain bike outside the shop Circus, at the junction of the Drapery with Gold Street.
A police spokeswoman, said: "He was approached by another boy who asked him for a cigarette and then threatened him with a knife before taking the bike and cycling off towards the bus station."
Witnesses or anyone with information about the incident can call police on 101 or Crimestoppers anonymously on 0800 111 555.