Social categorization under minimal group conditions reliably produces intergroup discrimination. It is proposed that this might be because the minimal group paradigm engenders high levels of subjective uncertainty among participants, which causes them to use the categorization to define self and thus identify with the minimal group. Uncertainty is generally an aversive state which may be resolved by identification (Hogg, 1996; Hogg & Abrams, 1993). Thus, people may identify with social categories (and express discrimination) when identification resolves uncertainty. To investigate this idea a standard minimal group experiment was conducted, in which the three independent variables of categorizations, task uncertainty and situational uncertainty were manipulated in a 2 x 2 x 2 factorial between-subjects design. Point distribution strategies were measured along with in-group identification, self-esteem, social awareness and, at three occasions, uncertainty about the task and situation. As predicted, under conditions of high task or situational uncertainty, categorized participants identified more with their minimal in-group and exhibited more intergroup discrimination than other participants. There was also some evidence that identification reduced uncertainty. There was only partial support for the mediational role of identification, and similarly, for self-esteem as a derivative motive. These findings are interpreted as supporting an uncertainty reduction model of social identification and group motivation.