Abstract. This study describes a teacher education experience with grade 5-6 teachers, based on a calculator module within a national program for mathematics in-service teacher education. The aim was to challenge the teachers' conceptions about the role of the calculator in mathematics teaching and to promote their reflection about professional practices. The research methodology was qualitative and interpretive, with data collection through interviews and observation of teacher education and classroom supervision sessions as well as analysis of teachers' portfolios. The results indicate that some teachers are clearly against the use of the calculator in the mathematics classroom, others allow students to use it in a passive way, and others are very affirmative about its use. The teachers who argue against the use of the calculator seem to predominate, suggesting a great distance between the curriculum orientations and classroom practice. The methodology of the course, combining collective sessions and individual classroom supervision proved to be fruitful, providing new information, practice and discussion that allowed teachers to analyze different kinds of tasks in which the calculator may be useful, experiment using then in the classroom, and reflect about the students' work. The no imposing and questioning approach used in collective discussions encouraged teachers to assume their own positions and sharing and discussing in the collective reflections during the course stimulated a deeper reflection of their practice. Therefore, in this course, in-service teacher education focused on practice contributed to teachers to reflect on their conceptions and practices.