Two case studies, both set in public high schools, one in New York City, another in suburban Massachusetts, describe ways that a newly developed cybertool (the electronic Teacher Guide or eTG) can help biology teachers improve student learning by facilitating teachers' implementation of an innovative curriculum and enhancing teachers' subject matter and pedagogical content knowledge. This tool consists of an eBook, a guide that describes the instructional sequence, an editable slide deck, web resources, and multi-media resources. These features enable teachers to implement and modify the curriculum with fidelity to the intentions of the developers and to enhance instructional practices such as conducting science discussions, implementing formative assessment tasks, and reflecting on their instruction. The case studies of teachers' use of the eTG revealed changes in practice along a continuum of novice to expert for multiple dimensions. Implications for designing and studying technology-supported educative curriculum are discussed.