The effects of ability grouping in schools on students' self-concept were examined in a sample of 23 secondary schools with a range of structured ability groupings. Measures of general self-concept, academic self-concept, and achievement were collected from over 1600 students aged 14-15 years and again two years later. Students' academic self-concept, but not their general self-concept, was related to the extent of ability grouping in the school attended. Subject-specific facets of academic self-concept were not related to the number of years of ability grouping students had experienced in English, mathematics and science; however, they were related to students' position in the grouping hierarchy, with students in high-ability groups having significantly higher self-concepts in all three subjects than students in low-ability groups. Students' intentions to learn in future were more strongly affected by self-concept than by achievement.