The frequencies per time unit of scent marking, aggressive threat, attacking another monkey and fighting it, were recorded in three Callithrix jacchus groups under different conditions: I-A: In un-disturbed situations. I-B: In the presence of strange males and immediately following encounters with these males. I-C: In the presence of strange females and immediately following encounters with these females. II-A: When a new perch was introduced into the monkeys’ cage. II-B: When a perch, scent marked previously by strange conspecifics, was introduced into the monkeys’ cage. The results, especially the role of pheromones in sexual, territorial and social behavior of Callithrix jacchus are discussed.