The purpose of this study was to use the Joint Action Studies in Didactics (JASD) to understand how teachers' and students' interactions co-construct knowledge during Cooperative Learning (CL). The basis of CL is that students learn with and from each other through a structured interdependent relationship. A case study approach was used to examine how a group of three year-5 students and their teacher from an ethnically diverse primary school in New Zealand co-constructed knowledge within a 3-month CL intervention in physical education. The JASD protocol included collecting data on the teacher's intention, practical epistemology and student interactions as they engaged in CL task structures. Data analysis included a search for patterns in the evolution of the didactic contract of content learned as students interacted in the co-construction of knowledge. Findings revealed that the CL tasks provided a pedagogical structure where student interactions were generally aligned with the didactic intent of the tasks. The teaching techniques of the student coach served to make the development of content more dynamic, with an increased frequency of breaches in the didactic contract. This dynamic may facilitate student learning during productive group problem-solving tasks. When the intended content was more specifically defined, the topogenetic technique of teacher intervention within tasks was critical to re-align students' interpretation of the knowledge at stake in the CL tasks.