Lesson study in Japan, the practice in which teachers work collaboratively and reflectively with colleagues on improving their classroom teaching, has recently attracted attention internationally as an effective means of teachers' professional development. Although lesson study is very useful for helping new teachers join the profession and for teachers to develop pedagogical knowledge, its effectiveness is less clear when teachers have to understand new epistemologies for new forms of learning and teaching. This paper describes a variation of lesson study, called design study, which takes the concept further to involve teachers and researchers working together as a design team. Specifically it describes the development of an experienced classroom teacher through his participation in a design-based research practice, tracing how he shifted his focus towards epistemological issues and successfully developed new pedagogical content knowledge. The paper describes how such a design-based collaborative study might enrich teacher knowledge, and articulates possible factors that might contribute to it as a successful practice. (Contains 5 tables and 1 figure.)