In this research, I collaborated with one fifth-grade teacher to integrate conceptual change instructional strategies into a revised unit on force and motion. Qualitative methods were primarily used to explore changes in the teachers' content knowledge, teaching beliefs, and classroom practices, as well as changes in students' knowledge. Sources of data included written assessments, teacher and student interviews and focus group, observations, and artifact collection. The teacher's colleagues also chose to participate in the revised unit, with strong impact on the unit as it was enacted. Findings indicate that the teacher's practices changed only slightly for some lessons; the majority of lessons retained her typical, text-driven practices. Students' understanding increased on topics the teacher taught conceptually, and did not increase on topics taught didactically. This lack of improvement in teaching practice is partially attributable to the influence of the teacher's colleagues, who opposed my efforts to replace the instructional materials and change classroom practices. I conclude by discussing the possible implications for professional development efforts and positing directions for future research in this area.