A 37-year-old man was left with a broken jaw after being assaulted in Duck Street, Rushden.
The victim was with two friends when he was approached by a man riding a mountain bike who stopped and assaulted him.
A second man on a bike arrived and then both the offender and he left the scene in the direction of Splash swimming pool.
The assault took place at 11.50pm on Saturday, near to the junction with High Street.
A police spokesman said: “The victim was taken to Northampton General Hospital where he received treatment for a broken jaw.
“Officers would like to speak to anyone who may have witnessed this incident or who may have any information regarding it.”
The attacker is a white man, aged between 18 and 24, 5ft 7in, of average build and wore a light-coloured hooded top.
The second offender is also a white man, in his 20s, 5ft 11in, of muscular build and wore a light grey top.
Any witnesses to the assault are asked to contact police on 101 or Crimestoppers anonymously on 0800 555111.