The social behavior in two small, polyandrous groups of moustached tamarins, Saguinus mystax, was studied at the Estacion Biologica Quebrada Blanco in northeastern Peru. Allogrooming was the most obvious type of social interaction. Grooming relations were characterized by a strong asymmetry. In both groups, one adult male contributed about 70% of all allogrooming given (in terms of time spent grooming) and allocated his allogrooming evenly to all other group members. Grooming relations between the second adult male and other group members were less intensive. Intragroup agonistic interactions were infrequent, and most of them occurred in the context of feeding on small food resources (fruit, exudate) and during intergroup encounters. Sexual behavior was rare, but at least in one group a tendency existed for the principal groomer to be somewhat more active than the other male. Results are compared with data from other field studies on callitrichid social behavior. Although the label poly- androus correctly describes the demographic structure of many groups, it is not clear whether it is sufficient for understanding the patterning of social relations and thus the social organization of tamarins.