This study examined teachers' content knowledge about the nature of science and how this knowledge was expressed in their classrooms. Understandings about science influence not only explicit lessons about the nature of science, but also shape an implicit curriculum concerning scientific knowledge. Consideration was also given to ways in which teachers' understandings about how students learn science in school influence teachers' beliefs and lessons about science. Seven precollege science teachers were interviewed to probe their knowledge about the nature of science. Three were asked to participate in the study based on their diversity of orientations. The teachers' knowledge about the nature of science, their roles as teachers, and their students' roles as learners were probed in a series of at least four hour-long, audiotaped interviews. Their classrooms were observed and audiotaped for at least 35 hours each over a 4-month period. During the interviews and the classroom observations the researcher was continuously involved in building, validating, and altering hypotheses concerning the teachers' content knowledge about the nature of science. The data from this study illustrates how teachers' views of science may be expressed in their classroom instruction. (MVL)