Two studies were conducted to test the hypothesis that information about intergroup competition is central to children's representations of social categories. Children (N = 99, 5- and 6-year-olds) were introduced to two novel social categories, which were described as having competing or noncompeting goals, by varying the quantity of a resource in which both groups were interested. When groups had competing (as compared with noncompeting) goals, children expected category membership to more strongly constrain prosocial and antisocial behaviors, viewed category membership as more fundamental to identity, were more likely to reference categories to explain behavior, and viewed categories as characterized by unique social obligations. Results further indicated that children reliably inferred when goals were competing versus noncompeting based on information about resource quantity. Implications for the conceptual systems that support the development of social categorization are discussed. (Contains 3 figures and 1 footnote.)