Our purpose was to investigate the efficacy of using reflective practice to guide our action research study of incorporating formative assessment into middle school science teaching and learning. Using participatory action research, we worked collaboratively to incorporate formative assessment into two instructional units, and then engaged in systematic reflection on our teaching practices so as to make instructional decisions that supported student learning. Data sources included audio-taped reflective teaching sessions, student documents, instructional planning documents, and reflective research journal. Findings indicated we initially held divergent ideas of formative assessment, but ongoing attempts to fully utilize reflective teaching sessions helped us come to workable terms for classroom practice. As a result of reflective practice we made timely curricular enhancements to address students' learning needs. Critical reflection on formative assessment outcomes was essential for evaluating our instruction and for enhancing curricula in ways that genuinely supported academic needs of individual students. (Contains 3 figures and 2 notes.)