Allomaternal behavior is defined as maternal-like interactions between an infant and some animal other than its natural mother. The occurrence of allomothering can have a profound influence on the life of the infant and on the animal with which the infant interacts. Allomaternal interactions were studied during the first 6 months of life in 50 Bolivian squirrel monkeys in two different social settings. Allomaternal interactions began during the first week of life. Sampling indicated that the infants spent 30% of their time on allomothers during the first 6 months of life. Dorsal clinging on allomothers occurred most frequently when the infants were younger and more dependent, and declined with the infant's growing independence. In addition, squirrel monkey infants nursed on allomothers, though the rates were lower than nursing on their dam. Four- to six-year-old young adult females accounted for most of the allomaternal interactions (53%), while adult females 7 to 9 years old provided 21% of the allomaternal interactions. A similar age-related pattern was found for allomaternal nursing; however, reproductive status had a profound influence. Those females with a reproductive failure for the year provided 86% of the allomaternal nursing bouts. Allomaternal interactions were not based solely on kinship relationships. Future studies need to be directed toward establishing the proximal causes of allomaternal behavior in the squirrel monkey. © 1994 Wiley-Liss, Inc.