The objective of this study was to determine the size and composition of a group of wild Callithrix jacchus Erxleben, 1777, a small neotropical primate from the North East Brazil. The study was carried out in Dois Irmãos Forest, Recife, Pernambuco, Brazil, in an area of 6ha where this social group of marmosets had its home range. A graded transects system was cut to provide the access and moving during the observation period. To provide an individual identification for the members of the marmosets group and to achieve the objective of this study, two captures were made, with a six month interval between them in a 50x50m² area at the nuclear range of the group's area. The animals were realeased after a maximum period of 24h after traping. We made, during the whole study period, direct observations of the social group which wasn't limited only to count and identification of the marmosets, but expanded in consideration on the ecology and behavior of the Callithrix jacchus group. The results show us that the size and composition of the study group was similar to others descriptions for the Callitrichidae. The group composition suffered a 30% "turnover" in a six month interval, and the adults particulary, had a big frequency of change. One pair of infants was seen in December 1986 - in the middle of the dry season - and another infant appeared in the study group in 1987, just after the end of the wet season. Some dietary items were fruits, leaf buds and also exudate from the trunks and branchs of some tree's species. Intergroup agonistic interactions were recorded during February 1987.