This report aimed to measure the impact of a unique professional development program entitled Project ASTER III (Active Science Teaching Encourages Reform) on teachers' self-efficacy and perceptions about inquiry-based science teaching. Project ASTER III enabled teachers to explore inquiry-based science teaching through exhibit-based hands-on/minds-on investigations at a science museum and to develop a science curriculum aligned with museum exhibits and state and national science education standards. Quantitative data indicated that teacher beliefs were positively and significantly impacted by the professional development program and confirmed that programs like ASTER III are effective but need to be provided on a continuous basis to reinforce these beliefs in the teachers. Finally, three themes emerged from the analysis of the qualitative data from the participant journals: (1) impact on teacher understanding of inquiry, (2) increased confidence about science teaching, and (3) benefits of collaboration. (Contains 4 tables.)