Language teachers are increasingly expected to develop teaching materials in many contexts. This trend necessitates greater understanding of how language teachers develop their professional practices as materials developers. Applying an ecological perspective in combination with Wenger's theory of Communities of Practice, the present study examines the developmental trajectories of language teachers participating in a materials development community. Adopting a narrative case study approach, this study collected interview data, policy documents, meeting minutes, and personal materials that four language teachers used for developing materials. The findings show that these language teachers' developmental trajectories as materials developers were not linear and were mediated by individual beliefs and policy requirements upon entering the community; their knowledge and skills and the power relations they experienced during the materials development process; and their beliefs and power relations at the end of the project. These findings suggest that language teachers need individualized support and resources at various points throughout their participation in a materials development community to develop their professional practices throughout the process.