Following the introduction of a National Curriculum for Science, all secondary science teachers in England need to be prepared to teach all aspects of a broad and balanced science curriculum. This is the second paper in which we explore science student teachers' subject knowledge development during a one-year postgraduate teacher preparation course. In this qualitative study we explore the role of university tutors and school-based subject mentors in science student teachers' subject knowledge development as perceived by student teachers, school-based subject mentors and university tutors. The findings reveal that student teachers are reluctant to use university tutors and school mentors for subject knowledge development because they are aware of their assessment roles. The role of the university tutor in subject knowledge development is perceived as one of facilitation and of developing student teachers' sense of professionalism. School mentors perceive that they do provide support for subject knowledge development and they are largely unaware of any potential conflict arising from their assessment role. The findings of the study are discussed in terms of the nature of student teachers' professional learning in communities of practice. The implications for mentor training programmes are considered. (Contains 4 tables.)