In this article, we present part of the results obtained in a study aimed at investigating training and investigative possibilities of cases of teaching in processes of teachers’ professional development (NONO, 2005). In this research, which was carried out during the period 2001-2005, four new Infant and Elementary School teachers analyzed five teaching cases and constructed a teaching case based on school situations they had experienced during their careers. Teaching cases have proven appropriate for use in teacher training, and for researching teacher’s learning process. In this article, we highlight knowledge about the teaching of mathematical contents shown by the new teachers, by analyzing a case of a Mathematics teaching situation. The results obtained indicate doubts, convictions, mistakes and contradictions that characterize their professional practice when teaching mathematical contents.