This study examined changes in middle school teachers' beliefs about inquiry, implementation of inquiry practices, and self-efficacy to teach science through inquiry after participating in a year-long professional development program. The professional development model design was based on Bandura's (1986) social cognitive theory of learning and literature on effective professional development. Using the Electronic Quality of Inquiry Protocol (EQUIP) before and after the program (Marshall, Smart & Horton, 2010), teachers' quality of inquiry instruction significantly increased across all 4 EQUIP factors. To assess teachers' inquiry teaching efficacy, the Teaching Science as Inquiry (TSI; Dira-Smolleck, 2004) instrument was administered 3 times to the teacher participants. Changes between time points of TSI administration were examined for personal self-efficacy and outcome expectancy across 5 essential features of classroom inquiry. Findings for the personal self-efficacy scale showed statistically significant gains from pre-Institute to 1-year follow-up for all 5 inquiry features. Teachers' outcome expectancy beliefs significantly increased across the program for 3 of the 5 essential features of inquiry. The results indicate the effectiveness of the professional development model at increasing the participants' inquiry efficacy as well as the quality of their inquiry instruction. Our study provides evidence to support the need to include collaborative practice-teaching and reflection opportunities within professional development models that aim to increase teachers' use of inquiry-based instructional strategies.