In this paper we present a professional development course designed to impact on teachers' mathematical knowledge for teaching fractions. The main features of the course are the use of i) problem solving activities related with mathematical knowledge for teaching fractions ii) peer discussions and iii) monitor's interventions focused on answering with questions. The objective of this paper is to present some insights on how this type of course may contribute to the development of teachers' knowledge for teaching fractions. To do that we analyze in depth the case of one teacher, using the recording of his work during the course (video, audio, and written documents), his answers to two questionnaires and his responses to an interview six months after the course. Results show how the course's features contributed to the development of this teacher's specific fractions knowledge, knowledge of fractions and students, and of fractions and teaching.