A CRUCIAL ASPECT OF TEACHER ACTIVITY UNDER INDIVIDUALLY PRESCRIBED INSTRUCTION (IPI) IN MATHEMATICS IS THE DEVELOPMENT OF INDIVIDUALIZED LESSON PLANS OR PRESCRIPTIONS. THE QUALITY OF THESE PRESCRIPTIONS IS A MAJOR DETERMINANT OF THE EXTENT TO WHICH INSTRUCTION IS ACTUALLY INDIVIDUALIZED AND THE EXTENT TO WHICH EACH PUPIL IS PERMITTED TO PROGRESS AT HIS OWN BEST PACE. THE EVALUATION OF TEACHER PERFORMANCE BASED ON A MODEL OF PRESCRIPTION WRITING SHOWS THAT INDIVIDUAL PRESCRIPTIONS VARY AMONG CHILDREN, THAT EACH TEACHER HAS DEVELOPED A PERSONAL STYLE, AND THAT MOST TEACHERS RELY ALMOST EXCLUSIVELY ON PRE-UNIT TESTS. THE RESULTS OF THE STUDY SHOW THAT IMPROVEMENT OF IPI REQUIRES TEACHERS TO HAVE (1) CURRENT, EASILY AVAILABLE, AND COMPREHENSIVE INFORMATION ABOUT EACH STUDENT, (2) A GREATER VARIETY OF ASSIGNABLE MATERIALS, (3) SPECIFIC DEFINITIONS OF THE TERMS "MASTERY" AND "SELF-DIRECTION" IN RELATION TO OPERATING PROCEDURES, AND (4) A RATIONALE BEHIND VARIATIONS IN PRESCRIPTIONS WHICH CLOSELY FOLLOWS EACH CHILD'S LEARNING NEEDS. A RELATED DOCUMENT IS ED 010 210. THIS PAPER WAS PRESENTED AT THE AMERICAN EDUCATIONAL RESEARCH ASSOCIATION MEETINGS (WASHINGTON, D.C., FEBRUARY 16-18, 1967). (TT)