In tamarins and marmosets, long-term stable sociosexual relationships are formed between heterosexual adults, but these "monogamous" relationships are often formed in groups that contain multiple adults of both sexes. The patterning of interactions during pair formation may therefore be shaped by this demographic profile. We evaluated the development of sociosexual relationships in six captive pairs of Wied's black tufted-ear marmosets (Callithrix kuhli) during the 80 days from the initial day of pairing. Social behavior, sexual behavior and activity profiles were recorded. Social behaviors, including allogrooming, grooming solicitation, and intragroup monitoring calls, increased across the four 20-day time blocks. Males were more responsible than females for maintaining intrapair proximity during the first 40 days of pairing. Females and males were equally responsible for intrapair proximity maintenance after this time. The highest rates of sexual behavior, including copulation and proceptive open mouth displays, occurred upon pairing and then decreased non-significantly over time. The results indicate that sexual relationships in callitrichid primates are not dependent on the prior existence of a social relationship between males and females. Higher rates of copulation, greater male responsibility for proximity maintenance, and male initiative in sexual interactions early in pairing are consistent with a male reproductive strategy in which male-male competition may be common. © 1995 Wiley-Liss, Inc.