This study investigates how the impact of developmental education is moderated by classroom composition. Drawing on a regression discontinuity design, we used data from 3,429 community college students, nested within 223 classrooms, to explore the extent to which classroom and instructor characteristics moderated the effect of developmental English students' performance in a successive, college-level English course. Our results suggest developmental English students' college-level course performance benefited from developmental education but was modified by classroom and instructor characteristics. On average, the impact of participating in the developmental English program was greater when developmental students were enrolled in classrooms that contained a higher proportion of developmental students. Achievement for developmental students was also enhanced when developmental students enrolled in classrooms taught by full-time instructors. Findings suggest that after underprepared students completed developmental English, classroom composition in the first, college-level English course had significant influence on developmental students' performance.