Minimal group studies are sometimes interpreted as showing that social categorization per se inevitably produces discrimination. Self-categorization theory clarifies this point, suggesting that a process of self-categorization must occur to transform an external categorization into an internalized representation. Hogg and Abrams suggest that the underlying motive for self-categorization is the reduction of subjective uncertainty. Two minimal group experiments employing different manipulations of uncertainty were conducted in which categorization and subjective uncertainty were manipulated in a 2×2 design. Across both studies, as hypothesized, intergroup discrimination only occurred when participants were categorized under conditions of subjective uncertainty. This was accompanied by enhanced group identification and elevated self-esteem. It is concluded that categorization per se is necessary but not sufficient for discrimination—people must self-categorize, and this is motivated by a need for subjective uncertainty reduction. Discrimination is not an inevitable outcome of categorization.