In this paper, the authors examine differences between school types in the uses of ability grouping, instructional differences, and relationship of ability grouping to student mathematics achievement. Specifically, they address the following questions with teacher reports of students' mathematics placement in middle school: (1) Does the use of ability grouping differ between charter and traditional public schools?; and (2) What is the relationship between ability group placement and students' mathematics achievement gains? The data come from surveys of teachers in charter and traditional public schools, located in urban, suburban, and rural contexts across 24 states. The schools all participate in the Northwest Evaluation Association (NWEA) assessment program and student achievement data in mathematics come from NWEA assessments. The data come from the What Makes Schools Work project to examine organization and instructional conditions in different types of schools. The findings reveal significant differences in the use of ability grouping in charter (CPS) and traditional public schools (TPS). For example, a greater percentage of CPS students are placed in both high ability groups (17% compared with 12% TPS students) and lower ability groups (20% compared with 13% TPS). Fewer CPS students are in mixed ability groups (20% compared with 51% TPS). Moreover, CPS student gains in each group were larger than those of TPS students in similar groups, and the gains of students in the high ability group were greater than those in the low ability group, contributing to increasing inequality over the school year. Few instructional differences among groups and between CPS and TPS teachers were significantly different, although further analyses are necessary.