The social behavior of the common marmoset has been well studied in captivity, but little is known about the social dynamics of this species in its natural habitat. Social relationships were studied in three polygynous groups of common marmosets, Callithrix jacchus, in northeastern Brazil. Breeding adults appeared to be the center of social life and were the most frequent grooming partners or nearest neighbors for most adult group members. The observations of unidirectional agonistic interactions suggest that breeding adults were also dominant over all other group members, but that neither sex was dominant over the other. The dynamics of within-group social relationships are likely to be important determinants in the reproductive strategies employed by marmoset females. © 1995 Japan Monkey Centre.