A 13-year-old girl was indecently assaulted in an alley between Poynters Road and Hadrian Avenue on Thursday.
The girl and a friend were walking home at about 3.30pm when they were approached by two boys on bikes.
One of the boys blocked the path of the victim and touched her bottom over her clothing before leaving the scene.
The victim made her way home where she told her parents and raised the alarm.
The offender is described as a mixed race boy, between 13 and 15 years old, with a small build and very short dark hair. He was riding a black BMX bike which had large plastic spokes on the wheels.
There is no description of the second boy.
Call Det Con Simon Day in confidence, on 01582 394449, or Bedfordshire Police on 101, or text information to 07786 200011.