Videotape and participant observation were used to document an American high school teacher workgroup's experience with collaborative teacher inquiry and to monitor changes in practice through two cycles of instructional planning, classroom implementation, and reflective analysis. Detectable changes in practice were observed, including a substantial improvement for two of the four teachers in fidelity of implementation of an instructional innovation. Results support claim that meaningful instructional changes are more likely when teachers work in job-alike teams, are led by trained leaders, use inquiry-focused protocols, and have stable settings in which to engage in the continuous improvement of instruction. (Contains 5 tables.)