Previous studies indicate adult spider monkeys (Ateles geoffroyi ) display marked sex differences, including some that may emerge early in life. However, the social behavior of juvenile spider monkeys and potential sex differences has not been adequately studied. Sex differences in the social behavior and proximity patterns of individually recognized juvenile black-handed spider monkeys (A. geoffroyi ornatus) were examined at El Zota Biological Field Station in Costa Rica. Both males and females exhibited sex-typical differences. Only females initiated grooming. Females centered interaction around their mothers and female peers, while males interacted more with adult and juvenile males. Juvenile males were also found in significantly larger parties than females, and were in parties containing adult males significantly more than females. The results of this study indicate that juvenile spider monkeys begin to exhibit sex-typical behaviors at an early age, and that these behavioral patterns prepare them for the social challenges of adulthood.