Elementary teachers are typically hesitant to teach science. While a limited knowledge of science content is a reason for this, limited science pedagogical content knowledge (PCK) has emerged as another reason in recent research. This study constitutes two case studies of a professional development program for elementary teachers involving mentoring by a university professor. The mentor took the role of a critical friend in joint planning and teaching of science. The study examines the nature of the mentoring relationship and reports the type of teacher learning that occurred, with a particular focus on the teachers' development of science PCK.