To transform the knowledge and skills they develop through teacher education programmes into successful instructional practices, preservice teachers should strongly believe in their capabilities. This qualitative case study, to fill a gap in the literature, provides a theoretically sound description of how a mathematics teaching methods course fostered the development of preservice teachers' self-efficacy to teach mathematics for conceptual understanding. Nine junior preservice teachers were interviewed three times throughout a mathematics teaching methods course to reveal detailed information about the sources provided for participants' self-efficacy. Data analysis was conducted to (1) identify the perceived influence of the methods course on preservice teachers' self-efficacy and (2) explain how the methods course created influence through the hypothesised sources of self-efficacy. Findings show that various components of the methods course, which were categorised under 6 major factors (i.e. "lecture hours," "group work," "feedback on group work," "peers' presentations," "assigned readings," and "examination"), were described as influences, and each component operated through at least one hypothesised source of self-efficacy (i.e. "mastery experience," "vicarious experience," "social persuasion," and "affective state").