Between 2015 and 2018, a Lesson Study (LS) project has been led within the 3LS Laboratory of Lausanne (Switzerland) dealing with the teaching-learning of natural science in 3-4 HarmoS (6-7 year-old pupils). During the first two years of that project, 12 different teachers were involved and supervised by two teachers-researchers facilitators (Research presented at Wals, 2016). Among these 12 teachers, one was able to participate during the totality of the first two years of training. In view of the skills he developed all along the LS process, it was suggested that this teacher co-leads the third-year LS process with one of the two previous facilitators.The present contribution focuses on further new data collected during the last part of the training related to the case study of this teacher.