Given the current emphasis on conducting high-quality experimental studies, it is becoming increasingly important for researchers to accompany their studies with evaluations of the "fidelity of implementation" of the experimental treatments. This article compares the form and extent of an experimental treatment to student learning. The study involved six middle-school physical science teachers and their students. Videotaped lessons were coded according to their alignment with the treatment's intended structure and processes; results identified a 0.71 correlation between teachers' enactment of formative assessment and student learning. The article suggests that the results of the study may have been due, at least in part, to the failure of many of the experimental-group teachers to implement the treatment as intended. (Contains 8 figures, 10 tables and 2 footnotes.)