How can the teacher bring about effective cooperative learning (CL) in multiethnic elementary classrooms? To answer this question we hypothesized that when the teacher stimulates pupils' helping behaviour (experimental group), this increases pupils' performance and CL motivation more than when the teacher lets pupils fend for themselves (control group). Subjects were 166 pupils from 10 schools. The results show that national pupils in the experimental group outperformed pupils in the control group and teams with low and medium prior knowledge performed better in the experimental group. Additionally, immigrant teams with high prior knowledge in the control group outperformed their low prior knowledge counterparts and had a higher CL motivation. Our results suggest that, next to the teacher's role, attention has to be paid to both the pupil background characteristics ethnicity and prior knowledge and the teacher's experience with CL.