Science teaching orientations, defined as teachers' knowledge and beliefs about the purposes and goals for teaching science, have been identified as a critical component within the proposed pedagogical content knowledge (PCK) model for science teaching. Because of the scarcity of empirical studies in this area, this case study examined the nature and sources of science teaching orientations held by four highly regarded secondary biology teachers. Data sources consisted of transcripts from four interviews, a card-sorting task, and classroom observations. Using a grounded theory framework, inductive data analysis led to the construction of a substantive-level theory for this group of participants. In regard to the nature of science teaching orientations, the use of central and peripheral goals, as well as the means of achieving these goals, was used to represent the complex nature of participants' science teaching orientations. The participants' science teaching orientations included goals related to general schooling, the affective domain, and subject matter, although the latter was not always a central component. In regard to the sources of teaching orientations, participants were strongly influenced by the classroom context and their beliefs about learners and learning; additional influences included prior work experiences, professional development, and time constraints.