Purpose: The study aims to understand what learning is evidenced regarding knowledge about the teaching of mathematics by five primary teachers when they reflect on research lessons. To do so, we used situations from the formal and informal reflection sessions that took place after teaching four research lessons. Design/methodology/approach: It is a qualitative and interpretative research. Data collection was done by participant observation with elaboration of a research journal, audio recording of sessions and a whole-group interview. Data analysis was made by content analysis based on the conceptual framework. Findings: The study shows the relevance of carrying out successive research lessons, followed by moments of formal and informal reflection, leading to improvements in relation to the way the task is proposed to the pupils, the representations used and the monitoring and questioning carried out by the teacher in the various phases of the lesson. Originality/value: In lesson study, very often, only one research lesson is held. This case shows the potential of successive research lessons, each of which is followed by moments of reflection, in promoting the development of teachers' didactic knowledge, namely regarding teaching practice, providing a more in-depth look at the way pupils work on mathematical tasks and the role of the teacher in promoting learning. Reteaching proved to be a valuable strategy to support teachers' reflections in order to improve the leading of pupils' work.