This study aims to understand the teaching practices of teachers in the 1st cycle of Basic Education when introducing the concept of fractions to their students. It seeks to answer the questions: 1) How does the teacher introduce the concept of fraction to his students? 2) How does the teacher explore fraction interpretations in his classes? 3) What difficulties does the teacher experience in teaching fractions? A qualitative methodology was adopted in a case study approach to analyze six observed classes of a teacher who participated in a collaborative work program, focused on teaching fractions. The results suggest some weaknesses in the teacher's mathematical and didactic knowledge about fractions, namely in the interpretations of fractions, their approach and articulation in mathematics classes.