This article reports on the mentoring of foundation phase teachers and their development of disciplinary-specific knowledge, particularly how the practicum provides a space for mathematics content knowledge (MCK) and mathematics pedagogical content knowledge (MPCK) for student teachers, given the generalist knowledge base of many primary school teacher mentors. Data was collected through semi-structured interviews with two Grade 2 mentors and focus group interviews with third-year student teachers in an urban university setting. A two-step process of analysis comprising open-ended coding, followed by a form of deductive coding, within a framework derived from literature on the development of MCK and MPCK, lead to two main findings. Firstly, mentors might have hindered student teachers' development of MCK and MPCK, thus impacting on the building of comprehensive conceptual models of good mathematics teaching practice. Secondly, the efficacy of the university-led training is questioned. We therefore recommend that greater emphasis be placed on the development of subject-specific mentoring practices, especially in scarce-skills areas such as mathematics, and argue for a set of standards for mathematics-specific mentoring practices in South African primary school teacher education.