This longitudinal investigation assessed the development and changes in preservice science teachers' conceptions of subject matter and pedagogy as they proceeded through their secondary science methods course, practicum, microteaching, and student teaching. Twelve preservice science teachers were asked, on four occasions each several weeks apart, to specify what topics make up their primary teaching content area and what a diagram of the content area with these topics would look like. Interviews were conducted following student teaching to assess changes in preservice teachers' knowledge structures and to clarify any patterns. Results were analyzed in terms of the nature of preservice science teachers' subject matter and pedagogy knowledge structures; the source of these knowledge structures; stability of knowledge structures during teacher preparation; and the relationship between the knowledge structures and the act of teaching. Among the findings were: preservice science teachers do not have well-formed or stable subject matter or pedagogy knowledge structures; the responsibility of stimulating students to reflect on their subject matter seems to be most appropriately placed within the domain of the science educator; and preservice teachers reported that subject matter structures were translated into classroom practice with ease. Twelve handwritten charts and diagrams of the process are included. (Contains 17 references.) (JDD)