This paper describes the development of a sixth-grade "Matter and Molecules" unit using a new curriculum development model based on conceptual change research, the field testing of this unit in 15 classrooms and the field test results. The development process and the resulting unit are contrasted with the unit's predecessor, the "Models of Matter" unit in a commercial sixth-grade text. The development process was based on an extensive program of research on student conceptions and classroom teaching using preclinical interviews, pretests, classroom observations, journals by collaborating teachers, postclinical interviews, and posttests. The development procedures also included a careful content analysis and extensive interaction with collaborating teachers who were part of the development team. It is argued that the procedures described in this paper constitute a workable alternative to present curriculum development procedures and that the alternative procedures are superior in two respects. First, these procedures make use of the methods and findings of recent research on teaching and on students' scientific cognition. Second, posttest results and interviews with teachers indicate that the new unit was demonstrably superior to its commercial predecessor in terms of the students' conceptual understanding and the teachers' professional satisfaction. (Author/CW)