The present study examined the effect of intergroup distinctiveness and group membership on evaluations of impostors. We predicted that ingroup members would be harsher than outgroup members on an impostor and that perceptions of intergroup distinctiveness would further moderate these evaluations. Specifically, we tested the social identity theory prediction that low intergroup distinctiveness would lead to greater derogation of the impostor (the ‘reactive distinctiveness’ hypothesis) against the self-categorization hypothesis that high intergroup distinctiveness would instigate more derogation of an impostor (the ‘reflective distinctiveness’ hypothesis). In this study, vegetarians (ingroup members) and meat eaters (outgroup members) were presented with a target claiming to be vegetarian, but caught indulging in a meat dish. We found that ingroup members derogated the impostor more and felt less pleased about discovering the impostor behaviour than did outgroup members. In line with the reflective distinctiveness hypothesis, the heightened derogation displayed by ingroup members only emerged when intergroup distinctiveness was high, an effect that was mediated by ratings of group identification. The discussion focuses on the different responses intergroup distinctiveness may evoke. Copyright © 2005 John Wiley & Sons, Ltd.