In this nested mixed methods study I investigate factors influencing preservice elementary teachers' adaptation of science curriculum materials to better support students' engagement in science as inquiry. Analyses focus on two "reflective teaching assignments" completed by 46 preservice elementary teachers in an undergraduate elementary science methods course in which they were asked to adapt existing science curriculum materials to plan and enact inquiry-based science lessons in elementary classrooms. Data analysis involved regression modeling of artifacts associated with these lessons, as well as in-depth, semester-long case studies of six of these preservice teachers. Results suggest that features of the existing science curriculum materials, including measures of how inquiry-based they were, have a relatively small influence on the preservice teachers' curricular adaptations, while teacher-specific variables account for a much greater percentage of the variance. Evidence from the case studies illustrates the critical impact of the preservice teachers' field placement contexts as an explanatory, teacher-specific factor in their curricular adaptations. These findings have important implications for science teacher educators and science curriculum developers, in terms of not only better understanding how preservice teachers engage with curriculum materials, but also how programmatic features of teacher education programs influence their ability to do so.