This study, carried out in the high school context, is the first direct test of Berdahl’s status theory of sex-based harassment. The theory covers not just male harassment of females, but female harassment of males and same-sex harassment. Participants were 771 males and 679 females, from Years 8 to 10, in five co-educational lower socioeconomic status (SES) Australian city schools, participating in a wider study of peer victimization. They indicated on a 5-point scale (from never to almost every day) how frequently they had experienced each of six sex-based harassment behaviors over the previous year, from same-sex and from opposite-sex peers, and responded to a question about sense of safety at school. Nonparametric analyses supported five of seven hypotheses derived from the theory: boys harassed others most often, girls were harassed most often, boy-to-girl harassment was the most frequent, girls harassed girls more than they did boys, and girl-to-boy harassment was the least frequent. However, contrary to the theory, boys’ same-sex harassment was no more frequent than that between girls, and girl-to-girl harassment was just as threatening to victims’ sense of safety as boy-to-boy harassment. The study largely supports Berdahl’s theory. The unexpected results can be understood in terms of the intimate nature of adolescent girls’ groups in high schools and their centrality for identity formation. In this context, girls are highly motivated to defend their status in terms of stereotypically feminine standards regarding appearance, sexual activity, and access to high-status boys. The theory implies that structural changes to reduce the salience of sex differences and sex stereotyping will be crucial to efforts to address sex-based harassment.