This research aims to understand how prospective teachers develop their knowledge about whole-class discussions during a lesson study. Following a qualitative approach, we focus on the work of three prospective mathematics teachers preparing, leading, and reflecting on wholeclass discussions. Data was collected through participant observation with field notes, audio recordings, and document collection. The results show that a detailed analysis of a lesson plan, previously prepared autonomously by a prospective teacher based on a guide, promotes a focused discussion and a collaborative environment. Additionally, being involved in each other's formative process, through lesson study micro-cycles, created opportunities for prospective teachers to analyze and discuss different classroom situations, which promoted the development of their knowledge. Finally, classroom experiences seemed to be fundamental for the development of prospective teachers' knowledge. They were able to put into practice the work carried out during the lesson study, which allowed them to link theory and practice, and reflect on their own practice. This research also underlines the importance of the teacher educator's role in creating fruitful opportunities for the development of prospective teachers' knowledge.