Two field studies investigated whether, as predicted by self-categorization theory (Turner, 1987), the relationship between comparative fit of an ingroup-outgroup categorization and group phenomena is mediated by depersonalization of self-perception, and moderated by category accessibility. In the first study participants were football fans, and in the second they were employees in an organization. In each study, two experimental conditions were created, whereby the accessibility and salience of the ingroup-outgroup categorization were varied. New measures of comparative fit and depersonalization were developed, based on meta-contrast ratios. Outcome variables were ingroup bias (Studies 1 and 2), ingroup entitativity, organizational citizenship behaviours, job satisfaction and turnover intentions (Study 2). Consistent with self-categorization theory, results showed (a) that comparative fit determined ingroup bias and other criterion variables through the mediating process of depersonalization, and (b) that this process was active only when the category was highly accessible. The moderational role of accessibility concerned the relationship between depersonalization and outcome variables, not the link between fit and depersonalization. Copyright © 2006 John Wiley & Sons, Ltd.