The social organization, spatial utilization and movement patterns of free-ranging desmans (Galemys pyrenaicus G.) were studied by radiotracking individuals in the French Pyrenees. During the period of study, May to July, single adult male and female desmans formed a pair and defended their shared territory from neighbouring conspecifics by vigilance and scent-marking behaviour. Other adults did not form pair bonds and remained solitary. In such pairs, the territory of the adult male was always larger than, and completely enclosed, that of the paired female. Neighbouring pairs occupied contiguous territories of similar length. The territories of neighbouring males overlapped to a small extent, but the territory of one male and that of a neighbouring female were never seen to overlap. Juveniles were observed to utilize the territory of resident pairs, prior to their dispersal. Paired adults and juveniles exploited their ranges on a regular daily basis, whilst those of solitary adults, being larger than those of the former, were utilized on a 48–hour basis. Paired males appeared to invest more time in defending the borders of their shared territory than did females which, in contrast, spent most of their time within the centre of the territory. Despite some degree of territorial overlap between neighbouring conspecifics, their mutual avoidance, achieved through a system of temporal range utilization, resulted in few agonistic encounters.