The article reports on a small-scale in-depth research study investigating formative assessment enacted and theorised from a sociocultural perspective within a part-time Professional Doctorate in Education (EdD) programme in an English university. Going beyond its conventional conceptualisation within psychological and motivational frameworks, formative assessment here encouraged students to view their learning as entailing the development of identities as researchers. The research adopted a case study approach, drawing upon participant observation, discourse analysis of online discussion forum and email feedback, and two series of student interviews. Although the practice of formative assessment remains problematic, students' responses suggest the value of tutor feedback, including its ontological dimension. Given the wide-ranging backgrounds of the learners who participated in this study, our findings suggest the relevance of a sociocultural view of formative assessment for supporting a more diverse doctoral student population. This leads us to argue for doctoral supervision to be conceptualised more firmly as a pedagogic relation in which formative assessment has a key role. (Contains 3 figures.)