This study extended over two academic years and involved six biology teachers and 199 students in nine slow-learner classes in four high schools. Classroom interaction was observed and recorded, then analyzed using the Flanders System of Interaction Analysis. Teachers were assigned specific roles of maintaining a definite interaction pattern or of changing the pattern over time in a specified way. The patterns of interaction were correlated with student achievement in concept formation, problem solving, and total performance, as measured by tests published to accompany the Biological Sciences Curriculum Study "Patterns and Processes." The nine classes were divided into three groups which were given different treatments. Results are presented showing changes in achievement related to different interaction patterns. The results indicate that problem solving and concept formation are influenced in different ways by interaction patterns. (EB)