A program involving two experimental fifth-grade groups, one of 15 boys, the other of 15 girls, used specifically developed instructional materials to capitalize on the different interests and characteristics of boys and girls and to determine the effect of sexual segregation on their achievement in language arts and science. A third experimental, mixed group also utilized team teaching and a wide range of new equipment, but the material was not designed specifically to interest either sex. Results were compiled by pre- and post-testing the students in the three experimental groups and in three control counterparts which had covered the same basic curricular areas in a traditional classroom setting. The "All Boys" and "All Girls" experimental groups made significantly greater gains than the mixed group or the control groups in both language arts and science. Within the experimental single-sex groups, boys still tended, as in non-segregated classrooms, to achieve at a higher level in science, and girls to achieve at a higher level in language arts. (The document includes outlines of the curriculum studied by the experimental groups, as well as test results, some participant reactions, and other related research findings.) (MF)