Monitoring activity of an animal in a system for housing freely moving animals including a plurality of animal ID stations, includes the steps of a) recording a first visit of said animal at a first of said animal ID stations, b) recording a subsequent second visit of said animal at a second of said animal ID stations, and c) determining a distance covered by said animal as the distance between said first animal ID station at said first visit and said second animal ID station at said second visit. With such simple means, parts of which are already present in e.g. most robot dairy stables, important information about activity of animals can easily be gathered.