The study examined the relations between academic achievement and self-concepts in a sample of 1,067 seventh-grade students from 3 core ability streams in Singapore secondary education. Although between-stream differences in achievement were large, between-stream differences in academic self-concepts were negligible. Within each stream, levels of students' achievement and their self-concepts were systematically related only when considered in the matching academic domain and the appropriate level of specificity. In English, lower achievers in the high-ability stream tended to underestimate their achievement, whereas higher achievers in the low-ability stream tended to overestimate their achievement. This pattern, however, was not evident in mathematics and the general academic domain. Taken together, the findings highlight the importance of considering the interplay of domain specificity and same-stream peers in academic self-concepts.