Among early adolescents (10-12 years) in the Netherlands, this study examined the academic self-concept in terms of the big-fish-little-pond effect (BFLPE). The BFLPE implies that students in classes where the average achievement is low will have a higher academic self-concept than equally achieving students in classes where the average achievement is high. The social comparison process assumed to underlie this effect was examined by focusing on classmates' average achievement and the perceived relative achievement position in the school class. It was found that the perceived class position mediated the relationship between classmates' achievement and the academic self-concept. In addition, the effect of classmates' achievement on perceived relative academic position was stronger in smaller compared to larger classes. Furthermore, it was investigated whether classmates' gender was differently important for social comparison processes in the academic domain. It turned out that students' academic self-concept was affected by the achievements of same-gender classmates. Achievements of opposite-gender classmates only had an effect on the academic self-concept when the number of these classmates was small. The findings of this study support the theoretical principles underlying the big-fish-little-pond effect and are relevant for debates about class sizes and the utility of academic selective schooling. (Contains 1 figure, 3 tables and 4 notes.)