The implementation of formative assessment strategies is challenging for teachers. We evaluated teachers' implementation fidelity of a curriculum-embedded formative assessment programme for primary school science education, investigating both material-supported, direct application and subsequent transfer. Furthermore, the relationship between implementation fidelity and teacher variables was explored. N = 17 German primary school teachers participated in professional development on formative assessment, N = 11 teachers formed a control group. Teachers' implementation fidelity was evaluated via classroom observations student ratings and an analysis of students' workbooks, focusing on the frequency and quality of intended formative assessment elements (assessments, feedback and instructional adaptations). Regarding direct application, treatment group teachers' implementation fidelity was high, with slight variations in quality. Regarding transfer, implementation fidelity was lower but teachers still implemented more formative assessment elements than the control group. Teachers' pedagogical content knowledge and their evaluation of the formative assessment intervention were associated with implementation success.