Reform based curriculum offer a promising avenue to support greater student achievement in science. Yet teachers frequently adapt innovative curriculum when they use them in their own classrooms. In this study, we examine how 19 teachers adapted an inquiry-oriented middle school science curriculum. Specifically, we investigate how teachers' curricular adaptations (amount of time, level of completion, and activity structures), teacher self-efficacy (teacher comfort and student understanding), and teacher experience enacting the unit influenced student learning. Data sources included curriculum surveys, videotape observations of focal teachers, and pre- and post-tests from 1,234 students. Our analyses using hierarchical linear modeling found that 38% of the variance in student gain scores occurred between teachers. Two variables significantly predicted student learning: teacher experience and activity structure. Teachers who had previously taught the inquiry-oriented curriculum had greater student gains. For activity structure, students who completed investigations themselves had greater learning gains compared to students in classrooms who observed their teacher completing the investigations as demonstrations. These findings suggest that it can take time for teachers to effectively use innovative science curriculum. Furthermore, this study provides evidence for the importance of having students actively engaging in inquiry investigations to develop understandings of key science concepts. (Contains 8 tables, 4 figures and 4 notes.)