A 31-year-old man had part of his ear bitten off in what police said was an unprovoked attack while he was walking home with a friend.
They were walking towards Queen's Park in Bedford when they were attacked by a group of youths and one girl in the early hours of Saturday.
Police later found the missing part of his left ear and the man had surgery to re-attach it.
All of the attackers in the incident were white and aged between 16 and 18.
Det Con Jason Wheeler said: "This was a totally unprovoked attack on a man who was just walking home with a friend and he will be permanently disfigured because of the incident."
Police are looking for anyone who saw the attack or who saw the group, who were sitting on a bench, beforehand.