Although teachers have opportunities to learn about many things through teaching experience, we know little about how they develop science subject matter knowledge in this setting. With both limited opportunities to learn science subject matter knowledge before becoming teachers and minimal science professional development available while working as a teacher, it is important to understand the extent to which elementary teachers develop science subject matter knowledge in their regular classroom practice and the factors that influence that development. In this longitudinal, mixed methods study we collected both quantitative and qualitative data before and following their final field experience, which was their first opportunity to have significant teaching experience. Findings suggest two important factors for subject matter knowledge development: time for considering science subject matter and a learning setting that values science. In contrast, indicators of learner capacity (i.e., prior knowledge) and time teaching the topics were not associated with teacher subject matter knowledge development.