Based on the assumption that much can be learned from the studies of experienced science teachers, the purpose of this study is to identify the knowledge base that contributed to the instructional representations demonstrated by four exemplary junior high school biology teachers. Results are intended to provide suggestions that would be useful to science teacher educators and science teachers. Multiple qualitative data gathering methods and triangulation were employed to enhance the validity of the findings. Analysis indicated that the knowledge base of the teachers' instructional representations consisted of five major categories: knowledge of subject matter, students, curriculum, context, and alternative representations. Each category had several chief subcategories. These categories overlapped and interacted with each other. The results indicate that it is important to provide science teachers with knowledge in the five major categories and that it is necessary to help teachers integrate the different categories of knowledge for teaching. Contains 26 references. (LZ)