The present Topic Classification System (TCS) developed out of a 7-year period of research and supplements an earlier system (Aschner, Gallagher, 1965). The purpose of the system is to indicate 1) level of conceptualization (data, concept, or generalization); 2) the style of thinking (activity, description, explanation, evaluation-justification, evaluation-matching, or expansion); and 3) the emphasis of the instructor on skill or content in classroom discussion. A study was conducted to use the TSC to compare teacher instructional strategy when variables of subject matter, teacher background, student ability, and concept to be taught are held constant. Subjects were six biology teachers and their high ability students using the Biological Science Curriculum Study (BSCS). Discussion sessions of each class were recorded for three consecutive days while the teacher was introducing the subject of photosynthesis. Each script was divided and classified by two staff members independently. Significant differences were found between teachers in goals and levels dimensions but not in style. Teachers talked most, but with substantial differences between teachers in terms of amount of student talk allowed. Those students showing greatest degree of verbal expression (more boys than girls) also showed greater proficiency according to grades and tests. The TCS was a useful analytic system for describing teacher-student interaction and comparison of various instructional patterns. (Implications for research and teacher training are discussed.) (JS)