This paper analyses two grade 4 mathematics lessons given in Switzerland and in Japan by student teachers (pre-service teachers) in the context of a project-based international exchange program. The lesson, initially planned together by the nine student teachers of the two countries, was finally realised in quite different ways in Switzerland and in Japan. Using the notion of levels of didactical codetermination, the analysis makes explicit the differences of the two lessons and identify cultural elements that shape such lessons.