The main ecological, social and behavioural features of the De Brazza's monkey are described. Radio-tracking techniques have been used. The species lives in small family groups including the adult ♂, the adult ♀ and their offspring. Home ranges vary from 4 to 10 ha, always located near the riversides, and day ranges from 250 m to 1010 m. Variations in range utilization may be related to the fructification of some tree species. Behavioral mechanisms of group cohesion and defence, together with protective types of locomotion, seem to be adapted to avoid attracting the attention of predators rather than distracting them. They combine to distinguish the De Brazza's monkey from sympatric Cercopithecus species.