The sociosexual behavior of a free‐ranging Cebuella pygmaea troop containing two adult males was studied throughout a postpartum periestrous period of its reproductive female. A clear‐cut male‐initiated six‐day behavioral estrous period occurred from the 13th through 18th day postpartum, with a two‐day peak of mating activity on the 15th and 16th days. Both adult males attempted to mate with the female, but the dominant male maintained exclusive mating access to her by guarding behavior and aggression toward the subordinate male. Estrus‐related changes in the daily activity pattern included constant following of the female by the male, increased huddling and grooming between the consorts, a decrease in infant carrying, and suppression of insect foraging in the consorting male. Behaviors seen only during the periestrous period included genital presenting by the female, intensive licking and sniffing of her genitalia by the males, female‐guarding by the dominant male, anogenital scent‐marking on the male's body by the female, tongue protrusion and “tongue vibrating” by the male, and copulations, play chasing, and “consort walking” by the couple. Within the Callitrichidae, genital presenting and tongue vibrating in sexual context have been observed only in Cebuella. Copyright © 1987 Wiley‐Liss, Inc., A Wiley Company