The purpose of this study was to investigate the extent to which a sophisticated computer assisted instruction (CAI) system could be implemented on the type of microcomputer system currently found in the schools. A method was devised for comparing CAI languages and was used to rank five common CAI languages. The highest ranked language, NATAL, Canada's National Authoring Language, was used as the basis for the design of a new, microcomputer-based CAI system named CASTLE (Computer Assisted Student Tutorial Learning Environment). Although primarily designed for trained CAI authors and researchers, the system has features that could assist the beginning author in creating CAI lessons and courses. The design specifications for the CASTLE language and the CASTLE support system were defined and a subset of the language and support system was developed on a Commodore CBM 8096 microcomputer system. The CASTLE system software was written in COMAL-80, and techniques of incremental compilation in an interactive and interrogative environment were used. The completed system was evaluated and recommendations made for further research and development. Two references are listed. Abstracts are provided in English and French. (Author/LMM)