Current reform efforts in the teaching of high school biology demonstrate the need for a synthetic treatment of prominent concepts. There exists insufficient research that delineates the global content understandings--in this paper designated subject matter structures (SMS)--of biology teachers; or that assesses whether these SMS do, in fact, translate themselves into classroom practices. The purpose of this investigation was to determine the nature of biology teachers' SMS and the relationship of these structures to classroom procedures. Case studies of five biology teachers from five different high schools in a small region of a western rural state were conducted utilizing interviews, classroom observations, anecdotal data, and analyses of instructional materials. In phase 1 of the study, initial interviews were held in order to construct an academic and professional profile of each teacher. In phase 2, each teacher was observed during 15 periods within the same semester. The data were analyzed qualitatively to determine which SMS the teachers demonstrated within the classroom teaching context. These observed-SMS were compared to the SMS provided by the teachers in post-observation interviews and to the SMS encouraged by the text. In general, teachers' SMS tended to be fragmented, indicating the absence of coherence necessary to the integration of biological concepts within classroom instruction. Mediating variables between teachers' SMS and their classroom techniques included teachers' intentions, knowledge of textbook content, pedagogical knowledge, student curiosity and level of interest, teacher autonomy, and time constraints. (40 references) (JJK)