This study examines the nature and implications of peer academic reputations in math and science classes for early adolescents' achievement beliefs and behaviors. The sample was 840 students (51% girls; 36% African American, 47% European American, 7% Latino, 6% Asian American, and 3% Other). About half the sample (47%) was from 27 fifth-grade classrooms in elementary schools and about half the sample (53%) was from 28 sixth-grade classrooms in middle schools. Peer academic reputations and student adjustment were assessed in the fall and spring of the school year. Peer academic reputation in the fall was associated with students' self-concept, worry, and engagement (but not intrinsic value) in the spring, controlling for fall levels. Peer academic reputation operated similarly across gender, ethnicity, and grade level. Thus, peers' opinions and expectations about each other's math and science achievement matter for the development of students' achievement beliefs and behaviors in math and science.