Classroom decision-making was conceptualized within the framework of person-environment fit. A longitudinal sample of 2239 sixth graders in 117 mathematics classrooms were surveyed. The findings include: (1) students typically report fewer decision-making opportunities than they think they should have in their mathematics classrooms; (2) students within a classroom tend to agree among themselves concerning decision-making prerogatives that actually do exist, but there is less consensus concerning prerogatives that should exist; (3) congruence on these "can decide" and "should decide" dimensions is associated with positive values and affect concerning mathematics as well as high effort and expectancies for success in mathematics; (4) congruence in mathematics classrooms is not related in the same way to similar outcomes in social and athletic activity domains; (5) congruence is inversely related to student misbehavior at school; and (6) these effects of decision-making congruence persist even after controlling for the level of actual decision-making opportunities in the classroom. Since past research has demonstrated that children's values and expectancies predict significant educational outcomes, the findings of this study imply that educators should work toward increasing their students' decision-making congruence in mathematics. Thirty-two references, four tables, and the study instrument are appended. (Author/HNS)