Effective professional development programs for science teachers provide opportunities for active learning and teacher self-reflection on beliefs about science teaching, learning, and practice. One model that fosters active learning and promotes reflection is collaborative curriculum development, in which teachers work together with university facilitators to create curriculum materials. We used a two-case study design to investigate how teacher collaborative curriculum "design" (the first part of development, in which ideas for curriculum are created) impacted participant professional learning during a five-day summer institute. Interview or survey data were collected from 41 secondary biology teacher participants in two summer institutes. Results indicated that teachers experienced shifts in their science knowledge, beliefs about science, beliefs about science teaching and learning, and in their science teaching practice. We concluded that the curriculum design process, which can occur in a relatively short time period, can foster meaningful, task-oriented collaboration. The collaboration process provides the vehicle for active learning, where teachers can reflect on their beliefs while applying new knowledge to the classroom. Recommendations for other professional development programs along with a discussion of the program's unique philosophy are provided.