We examine the notion of impostors within groups, defined in this paper as people who make public claims to an identity while disguising their failure to fulfil key criteria for group membership. In Experiment 1, vegetarians showed heightened levels of negative affect toward vegetarians who ate meat occasionally compared to an authentic vegetarian. In contrast, non-vegetarians saw the impostor to be marginally more likeable than the authentic vegetarian. In Experiments 2 and 3, participants evaluated only a vegetarian who ate meat. Evaluations of the target were influenced by group attachment, such that participants who identified strongly as vegetarians downgraded the target more strongly and experienced more negative affect than did moderate identifiers and non-vegetarians. Participants were also sensitive to the size of the gulf between the target's claims for identity and their behaviour. Thus, targets who made public claims to being a vegetarian but ate meat were evaluated more negatively than were people who kept their claims for identity private (Experiment 2). Similarly, targets who tried to keep their deviant behaviour secret were evaluated more negatively than were people who openly admitted their deviant behaviour (Experiment 3). The reasons why impostors might threaten the integrity of group identities are discussed. Copyright © 2003 John Wiley & Sons, Ltd.