The factors influencing the effectiveness of co-operative learning in an integration context involving average students and mentally retarded students are still widely unknown. With the goal of enriching knowledge in this field, we made a didactic analysis of a co-operative learning situation in mathematics with the help of Brousseau's "theory of didactic situations" and Artigue's model of "didactic engineering". The learning situation deals with measurement. It was designed and applied by elementary school teachers in the framework of a pragmatic-interpretative action research. This analysis brought out a number of conditions that could promote the emergence of unilateral or overprotective behaviours, which could cause dependence or inertia in mentally retarded students and which could be quite harmful to the search for collaboration among group members. It also revealed an absence of inductive questioning on the part of the teachers.