A contribution to the knowledge of the social organization of a population of Golden-rumped or Black-lion tamarin (L. chrysopygus) living in a stripe of natural vegetation in the State Park along the banks of the lower Paranapanema River, in the State of São Paulo, Brazil. Its composition and analysis based on captured groups, number of individuals and their distribuition in the area with concentrations; exchange of adults males, migration of individuals from groups and females home range establishment, indicate an organization which is likely to be matrilinear. The average number of individuals per group or expanded families, in the seven (7) captured groups was 3,57 specimens with a mean body weight in adults of 572,5 grams. In each group there is normally one reproducing couple with a double twin pregnancy in 2/3 (65 to 80%) of the occasions, and the twins are dizygotic. This tends to repetition in multiparous females.