This study examines the impact of standards-based professional development on teacher efficacy and instructional practice of elementary and middle school science teachers. Professional development activities were conducted over a period of 3 years and included content courses as well as teacher involvement in professional learning communities. Teacher efficacy was assessed at five time points using the Science Teacher Efficacy Beliefs Instrument (STEBI), and instructional practice was evaluated at four time points with classroom observations using the Reformed Teaching Observation Protocol (RTOP). Results indicate that there was significant growth in teacher self-efficacy but not in outcome expectancy. There was also significant growth in the extent to which teachers implemented inquiry-based instruction in the classroom. Additionally, a positive correlation was observed between changes in self-efficacy and changes in the use of inquiry-based instructional practice. These results are discussed in terms of a model of teacher learning wherein changes in teacher beliefs and changes in classroom practice mutually influence each other, and also in terms of the impact of collaboration on teacher efficacy and practice. (Contains 3 tables and 5 figures.)