The design, implementation, and evaluation of instructional radio used to teach primary school mathematics in rural Nicaragua is described in detail. The purpose of this Agency for International Development (AID) research and development project has been to demonstrate that rural school children can receive consistently high quality instruction in mathematics by radio. The first- and second-grade mathematics achievement scores reported indicate that the methods used have significantly improved radio's performance as an instructional medium. An extensive analysis of data collected and its interpretation is reported. (MP)