This study used a mediational model to examine the effect of curriculum condition on student behaviors and how those behaviors related to math achievement gains. The student behaviors examined were children's participation in learning-related activities, talking during learning activities, and level of engagement with the materials. Conducted as part of a larger project involving the implementation and evaluation of a preschool mathematics curriculum, this study involved twenty urban prekindergarten sites with 57 classrooms randomly assigned to the new curriculum or a practice-as-usual condition. The final analytic sample included 675 children, predominately from low-income homes. Results showed that the greater gains made by children in curriculum classrooms were mediated by the number of times children were observed focused on mathematics and the number of times they were observed talking during mathematics. Findings from this study demonstrate the importance of focusing on children's actual responses to a new curriculum in determining its effects. (author abstract)