This study examined the contribution of classroom format on teaching effectiveness and achievement in English language arts (ELA) and mathematics. Secondary data analyses of the Measures of Effective Teaching database included 464 US classrooms. Classrooms were defined as self-contained if a generalist teacher provided instruction on all subjects and departmentalized if a specialist teacher provided instruction on a specific subject. Beginning-of-the-year classroom-level covariates were compared. Both ELA and mathematics self-contained classrooms had larger class sizes, served more students of color, served students with lower initial achievement, and had teachers with fewer years of teaching experience but more likely to have a Master's degree. Regression models were used to determine if classroom format predicted teaching effectiveness and achievement while controlling for beginning-of-the-year classroom-level covariates. Departmentalization had a small positive association with higher teaching effectiveness ratings in ELA classes. Classroom format was not a significant predictor of achievement in ELA or math.