The costs and benefits of territorial defense were examined in a group of five wild golden lion tamarins, Leontopithecus rosalia, at Poco d'Antas Biological reserve, Rio de Janeiro, Brazil. I examined the effects of both interference and exploitative competition between groups of tamarins by comparing their use of space, time budgets and foraging success in different contexts of intergroup interactions and in quadrats shared and not shared by other groups. Tamarins spent more time moving and vocalizing, and less time feeding, foraging, and resting during intergroup encounters than in non-encounter contexts. Irrespective of intergroup distance, the group spent more time in overlapping areas of the range periphery than expected on the basis of quadrat availability. In those areas, however, foraging success per unit of foraging effort was lower than in exclusive areas of the range center. This suggests that access to high payoff central areas depended on costly defense (by both interference and exploitation) of the range periphery. Time and energy invested by the resident group in territorial defense (1) increased the availability of food in the range center, and (2) minimized food loss to neighboring groups in the range periphery. These benefits are likely to justify the costs of defense for an animal which depends on easily-depletable food supplies, such as prey in microhabitats and small concentrations of fruits and nectar.