The purpose of this investigation was to determine the nature of biology teachers' global content understandings (herein called subject matter structure-SMS), the sources and formation of SMSs, and the variables that differentially affected teachers' abilities to translate SMSs into classroom practice. Case studies of 5 experienced biology teachers were constructed through interviews, classroom observations, and analysis of instructional materials. The data were qualitatively analyzed to describe the SMS exhibited by the teacher in the classroom and compared to SMSs provided by the teachers in postobservation interviews. The teachers' SMSs for biology were based on discrete content topics rather than conceptually integrated themes. Though most teachers recognized the integrated nature of biology, few used such conceptions to guide practice purposefully. The initial formation of SMSs were typically credited to college content courses and modified by the act of teaching. Opportunities for reflection and reinforcement seemed critical for the formation of coherent SMSs. The relationship of SMSs to classroom practice was complex and varied. The most direct form of translation occurred in the scope of course content. Variables that differentially affected SMS translation (typically through mitigation) included teacher intentions, content knowledge, pedagogical knowledge, students, teacher autonomy, and time.