Attending academically selective schools is intended to have positive effects, but a growing body of theoretical and empirical research demonstrates that the effects are negative for academic self-concept. The big-fish-little-pond effect (BFLPE), based on social comparison theory, posits that equally able students will have lower academic self-concepts in academically selective schools than in non-selective schools. Here we test the validity of these predictions for representative samples of 15-year-olds from eight Australian states and territories by using multi-level modelling. Consistent with the BFLPE, the effects of individual student achievement were positive but the effects of school-average achievement were negative. Although there were small differences between states/territories in academic achievement, there were no significant differences between states/territories in the negative effects of school-average ability. (Contains 4 tables and 3 figures.)