This study focused on how science teachers' content knowledge influences their pedagogical content knowledge (PCK) with respect to the topic of cell division. The data were collected from 3 in-service teachers using interviews; observations; and teacher-provided documents, including written-answer exams. The participants' content knowledge was analyzed by inductive coding with respect to conceptual understanding, whereas their PCK was analyzed by within-case analysis followed by cross-case analysis to show how content knowledge affects PCK. The findings showed that content knowledge may support knowledge of what students understand in science as well as knowledge of instructional strategies, whereas the impact of content knowledge on teachers' knowledge of the science curriculum and their knowledge of assessment in science is complicated. Moreover, content knowledge may not have any effect on teachers' orientations toward teaching science. Implications for science teacher education and research are discussed.