In this article, the author examines a preservice teacher’s use of the Inquiry-Application Instructional Model (I-AIM) to plan and enact an instructional sequence about photosynthesis. The author concludes that Leslie’s case illustrates some of the mediators that may shape preservice teachers’ uses of an instructional model, including their approach to teaching science, the curriculum materials they have available, and the meanings they make of the key constructs underlying the model.