In this study, the implementation effects of a staff development program on cooperative learning (CL) for Dutch elementary school teachers were studied. A pre-test-post-test non-equivalent control group design was used to investigate program effects on the instructional behaviours of teachers. Based on observations of teacher behaviour during cooperative lessons, a statistically significant treatment effect was found for the following instructional behaviours: structuring positive interdependence, individual accountability, social skills and evaluation of the group process. Training effects were also found for the combination of CL with the model of direct instruction and the activation of prior knowledge of social skills. Moreover, teachers in the experimental group scored statistically significantly higher than the teachers in the control group on the activation of prior academic knowledge. (Contains 6 tables and 1 note.)