This study examined situational fluctuations in student teachers’ (N = 120) development in self-efficacy during a three-week teaching practicum in Switzerland. Situational measurements (state) were assessed during a six-lesson teaching unit on written argumentation. Results showed that student teachers’ intra-individual state self-efficacy increased during the practicum. However, multilevel regression models indicated that student teachers’ state self-efficacy fluctuated considerably during the teaching unit with approximately 30% of variance being located at the lesson level. This variation was predicted by mastery experiences (i.e., experience of competence and instructional quality) and cooperating teachers’ discourse elements in lesson conferences (i.e., co-constructive planning, student teacher orientation).