It was hypothesized that, in natural group contexts, low-status in-group membership would be highly accessible, whereas membership to high-status groups would not. Therefore, gender group membership was predicted to be more accessible for women than for men. It was further hypothesized that the high accessibility of gender group membership would lead to stronger self-stereotyping for women than for men. To measure the accessibility of gender group membership, participants performed a Gender Self-Categorization Implicit Association Test (Studies 1 and 2), measuring the strength of automatic associations between the self and the gender in-group. Participants also performed a Self-Stereotyping Implicit Association Test (Study 2), assessing the strength of automatic associations between the self and the stereotypical traits of the in-group. As expected, implicit gender self-categorization and implicit gender self-stereotyping were stronger for women than for men. Importantly, implicit gender self-categorization mediated the relation between gender and self-stereotyping. Therefore, implicit gender self-categorization was the mechanism underlying stronger implicit self-stereotyping by women. Copyright © 2012 John Wiley & Sons, Ltd.