Background: Curriculum fidelity describes the extent to which a curriculum is implemented faithfully as planned. Curriculum fidelity issues may arise when teachers implement the curriculum inconsistently due to differences in philosophy, barriers in the setting, or other local concerns. Purpose: The study examined challenges that a teacher faced in implementing a constructivist physical education curriculum that had fidelity implications. Research design: Ethnographic case study design was employed in the research. Participants and setting: One physical education teacher, "Daniel", and his students in the third, fourth, and fifth grades participated in the study as they were involved in a curriculum intervention in a large urban school district in the US. Daniel's school was randomly assigned to an experimental group to implement a physical education curriculum based on health/fitness-related science. Data collection: The researchers observed 75 lessons taught by Daniel using non-participant observation techniques and conducted two structured interviews with Daniel and eight interviews with his students. Data analysis: Constant comparison with open and axial coding was used to analyze the observation and interview data. Findings: Two thematic challenges emerged: (1) school contextual constraints that limited the fitness science learning environment in physical education, and (2) Daniel's personal values and preference for a recreational rather than a science-based physical education program. These challenges impacted Daniel's decisions when teaching the curriculum. (Contains 1 note.)