The Oum Er Rbia river in Morocco is particularly threatened by lack of appropriate management. It is in this context, this study of the biological diversity of benthic macroinvertebrates in the middle zone of Oum Er Rbia river and the evaluation of the biological quality of water at this level, during a campaign carried out in April 2018. The results show that the stations upstream of the two sources of agricultural and industrial pollution represented by the stations (S1, S3, S4, S5 and S6), the biodiversity is important and the water is of good quality. So that the discharge of wastewater from the city of Kasba Tadla (S2) and Dar Ouled Zaidouh correspond to the station S7 has caused a reduction in diversity and a degradation of water quality of Oum Er Rbia river. We also noted a significant improvement in water quality away from Dar Ouled Zaidouh by 5 km from sources of pollution, which demonstrates the purifying power of the water of the Oum Er Rbia river.