In this study, the authors assessed the contribution of engagement (on-task behavior) to the mathematics problem-solving performance of seventh-grade students after accounting for prior mathematics achievement. A subsample of seventh-grade students in four mathematics classrooms (one high-, two average-, and one low-achieving) from a larger intervention study assessing improvement in middle school students' proportional reasoning was assessed on initial mathematics achievement, on-task behavior, and mathematics problem-solving performance. Results suggested that engagement uniquely predicted mathematics problem- solving performance after controlling for prior mathematics achievement. Furthermore, the authors found differential rates of engagement for the three achievement groups. Based on an analysis of engagement by instructional lesson, the authors offer suggestions for addressing engagement when designing instruction. [This article was published in "Preventing School Failure" (EJ1090266).]