Differential effects of a peer-tutoring intervention on the academic achievement of 203 7th-grade science students with and without disabilities in co-teaching and non-co-teaching settings were examined over an 8-week period. Impact of peer tutoring was assessed using a 2 condition by 2 settings by 2 types of students analysis of covariance with pretests as covariates. Results indicate that the peer-tutoring intervention was associated with improvements in student performance, and students in co-teaching settings perform better than those in non-co-teaching settings, but no additional value was added when peer tutoring was implemented in co-taught classrooms. Because co-teaching is increasingly used for teaching students with disabilities in general education classroom settings, implications of these findings for current practice and the need for future research are discussed. (Contains 3 tables and 1 figure.)