The study reported in this paper builds on a broader work aiming to identify factors that affect the development of prospective primary teachers' efficacy beliefs (TEB) with respect to teaching mathematics. Eight student teachers were interviewed in the beginning, in the middle and in the end of a fieldwork course; they were encouraged to reflect on their experiences throughout the course. The analysis of the data revealed that mentors could influence student teachers' beliefs through their own teaching style, the feedback they provide to students, and the latent messages they implicitly convey to students. Yet, mentors' influence on prospective teachers' TEB should be regarded as a function of variable factors including the student-mentor personality matching. Implications for developing practicum courses and for further research are drawn. [For complete proceedings, see ED496851.]