This study addresses several methodological problems that have confronted prior research on the effect of class size on student achievement. Unlike previous studies, this analysis accounts for the hierarchical data structure of student achievement, where grades are nested within classes and students, and considers a wide range of class sizes across various disciplines. Based on data from undergraduate class sections at a single institution, the study provides consistent evidence of a negative effect of class size on grade performance, most substantially affecting the achievement of "A" grades with lesser effect on grades of "C" or higher. Because models with logarithmic specification show superior fit, this study also demonstrates that the effect of class size on students' final grades diminishes as the class size increases. These findings suggest that a larger impact on student performance could be attained by further trimming enrollment in small classes than by reducing class size overall. (Contains 7 tables and 4 figures.)