Long-term recordings of the locomotor activity of 9 common marmosets, Callithrix j. jacchus, show that under certain preconditions, social entrainment of the free-running circadian rhythm can occur in these non-human primates. However, there exist considerable interindividual differences in the Zeitgeber effect of the various forms of social contact. Findings indicate that both the level of acquaintance and the individual differences in spontaneous period of individuals involved are important, and that daily, or circadian, social cues have a cumulative effect. Because of the low Zeitgeber strength of social stimuli, however, desynchronization of different individual rhythms can arise spontaneously among animals kept as a pair in the same cage, even after months of harmonized free-running rhythms. It is discussed that social entrainment in primates and other mammals is more likely to be a coincidental, rather than an essential, characteristic of the circadian system. Arousal triggered by social stimuli is thereby ascribed a key role.