A man was hit with a plank of wood after being approached by a group of about 15 teenagers in Wellingborough.
The 21-year-old victim was in Croyland Gardens just after 5pm on Friday, May 26, when the incident took place.
One of the teenagers struck him on the back with a plank of wood.
Three or four other members of the group also hit him on the shoulder and back, causing pain and bruising which required medical treatment.
The group were all white and about 17 years old.
One of the offenders had blond hair.
Anyone with information can contact police on 101 or Crimestoppers anonymously on 0800 555111.