Prospective teachers perceive peer coaching as a supportive learning opportunity providing benefits in peers' engagement in active learning, communicative competences, and critical thinking about teaching. Little is known about whether peer coaching also promotes the development of pedagogical content knowledge (PCK) in teacher education, which is regarded as a central dimension of teachers' professional knowledge. We used a mixed-methods design to investigate whether Content-Focused Peer Coaching (CPC) supports pre-service biology teachers in planning lessons for teaching scientific methods. Pre-service biology teachers (N = 65) from three Swiss and one German university were trained on CPC. After the training, the peers were asked to plan a lesson on teaching scientific methods and to refine their lesson plans in a CPC session. The participants of a control group (N = 53) were given the same task but received no training for CPC. Then, 118 peer dialogs were videotaped, transcribed and analyzed together with the lesson plans (including objectives, timing, instruction materials, procedures and evaluations) via qualitative content analysis focusing on PCK. The results show that the peers of the intervention group discuss a wider range and a larger number of PCK issues than the control group and record more changes in their planning documents. However, the results also reveal that important PCK issues are rarely addressed in the peers' lesson-planning dialogs (e.g., dealing with students' conceptions and reflecting on lesson processes and results). Therefore the potential of peer coaching approaches to develop PCK might be limited.