This article focuses on a cooperating teacher in Norway and her approach to mentoring first-year student teachers in their process of planning the mathematics teaching of third-graders. The purpose of the text is to show how the cooperating teacher's mentoring assists the student teachers' performance in a teaching form that is acknowledged as difficult for novice teachers. Being engaged in such teaching forms is at the same time perceived as necessary in order to develop pedagogical content knowledge. I argue that the cooperating teacher has a key role as mentor for student teachers in this process. (Contains 2 footnotes.)