A 17-year-old boy was punched in the face and robbed of his iPhone 5 in Aylesbury on Friday night.
The victim was walking towards the Oxford Road underpass, leading to Wycliffe End, when he was approached by two men.
One of the men punched him in the face and stole his iPhone 5.
The two men were then joined by a third man who was on a bike.
The victim walked off towards Wycliffe End.
The incident happened at about 8.30pm.
One of the offenders is in his 20s, slim, between 5ft 9in and 5ft 11in and was wearing dark jeans and dirty trainers.
The second was similar in age and build and was wearing unbranded trainers.
The third man, who was on the bike, was stockier than the other two and was wearing a hooded jacket.
Anyone with information should call police on 101 or Crimestoppers anonymously on 0800 555 111.