Five teenagers were threatened with a knife in a robbery on playing fields in St Crispin, Duston, Northampton.
The incident happened between 7.45pm and 8.30pm on Wednesday, while the boys were playing on the field. They were approached by two males who threatened them and stole mobile phones and money.
One of the men is described as black, aged 18 to 21, tall and skinny, with a goatee beard. He was wearing black trousers and a black jacket with the hood up.
The second man was also black, aged 15 to 18, small with an average build. He was tracksuit top and bottoms and also had his hood up.
Both suspects were riding bikes.
Witnesses, or anyone with information, are asked to call Northamptonshire Police on 101. Alternatively, they can call the independent charity Crimestoppers anonymously on 0800 555 111.