A concern about grammar and usage instruction is the ineffectiveness of the frequently-used practice of worksheets and out-of-context application. In light of recent findings that applied grammar instruction can improve students' writing, a need for research on increasing teachers' pedagogical knowledge in this area is indicated. This case study examines teacher professional development to consider what aspects promote and extend teacher knowledge and practice in this area. Three high-school English teachers participated in a month-long Lesson Study cycle. Findings suggest that teachers' insights about active, collaborative grammar instruction appear to have been supported by their own collaboration and by joint observation of practice. Providing opportunities for teachers to reflect together on what was observed encouraged learning that transcended the specific lesson.