This study examined sex differences in three types of workplace aggression previously identified by Baron et al. : expressions of hostility, obstructionism, and overt aggression. Males reported engaging all three types of workplace aggression more often than females. The magnitude of the sex differences was nearly identical across aggression types. Path analyses revealed the relationship between sex and expressions of hostility was mediated by respondents' expectancies about the potential costs and benefits of engaging in this type of aggression. Expected benefit was the sole mediator of the sex-obstructionism and sex-overt aggression relationships. Finally, perceived self-control over aggressive impulses significantly predicted all three types of aggressive behaviour, but it was unrelated to sex and therefore did not operate as a mediator. Aggr. Behav. 00:00–00, 2005. © 2005 Wiley-Liss, Inc.