Recent research shows that students' understanding of many scientific topics is influenced by misconceptions that conflict with accepted scientific theories and persist even after instruction. In this study, student misconceptions were identified and used as a basis for analyzing the classroom behavior of teachers and students and for developing modifications that increased the effectiveness of commercial science programs. Results, obtained from case studies of teachers using either the light unit from Laidlaw Brothers "Exploring Science" textbook or the Producers part of the Science Curriculum Improvement Study (SCIS-II) "Communities" unit, indicated that: most students had misconceptions; the teachers exhibited a variety of teaching styles that did not take student misconceptions into account; and fewer than one quarter of the students learned the scientific conceptions they studied. When teachers used modified materials (which informed them about likely student misconceptions and suggested strategies for helping students to change), important changes in the teachers' behavior were noted and at least three times as many students understood the scientific conceptions of light than in the previous year. Learning for the Producers part did not improve, but other problems with the part were identified. When these problems were addressed in a subsequent study, learning improved substantially. (Author/JN)