This study investigated variation in students' behavioral engagement across mathematics classes in China and the United States. Student behavioral engagement was examined along with two aspects of the classroom (group size and teacher instructions given about classroom behavior). Video observational data were collected and coded over 1051 time intervals in 35-minute mathematics sessions in Chinese classrooms (n = 8) and comparable American classrooms (n = 7). Latent growth analyses revealed that overall, behavioral engagement declined over time, although the drop-off was dramatically sharper in American classrooms relative to Chinese classrooms. In addition, larger group size and the timing of teacher instructions (given before versus after the behavior) were significantly associated with increased engagement. This study revealed compelling cultural differences as well as patterns in student and teacher behaviors associated with students engaging in on-task behaviors in the classroom. Implications for ways to promote effective classroom behavior are discussed. (Contains 1 figure and 3 tables.)