This paper investigates the impact of a two-year professional development program on teacher self-efficacy for inquiry-based instruction. The program utilizes scientist-teacher partnerships to develop content knowledge, inquiry-based instruction, and leadership skills for in-service STEM teachers. Participating teachers are administered a survey to assess their self-efficacy at the start and completion of the program. The findings suggest increases in self-efficacy for inquiry-based instruction and greater focus by the teachers on the depth of content after completing the program. The results substantiate prior research conducted on the benefits of teacher participation in professional development utilizing a scientist-teacher partnership model. Other trends in the data, future research, and implications for the design or refinement of professional development programs are discussed.