This research focused on an institutional structural process and its relationship to sex differences in mathematics achievement. Instructional grouping by class and school interacted with curriculum and achievement. The sample consisted of 906 students in grades kindergarten through sixth in two elementary schools. Results showed that primary grade girls scored higher (p < .01) in a program with continuous student progress, individualized or modified group settings, pacing set by criterion tests, and a sequential, coordinated math curriculum. Lowest female achievement occurred in the traditional program with whole class instruction, adherence to grade level material, and little coordination between grades. Boys achieved equally well in both schools. (Author)