Supporting preservice teachers in their development of positive mathematical identities is important because of their future responsibility as teachers. In this self-study, we investigated a mathematics teacher educator's facilitation practices during discussions to examine opportunities for preservice teachers to develop productive mathematical identities. We analysed over 35 hours of video recordings of a mathematics content course for future elementary teachers and identified four key mathematics teacher educator facilitation practices that appeared to support preservice teachers to develop productive mathematical identities: emphasizing reasoning, promoting broader engagement, shifting responsibility for learning, and developing a supportive classroom community. We also analysed survey data of our preservice teachers' perceptions of their mathematical agency and authority. Implications for mathematics teacher educators are discussed.