To estimate the quality of the Soueir wadi, a small river located in the Drader - Soueir basin (In North-western Morocco) we undertook a physicochemical and biological study. This work done on River “Soueir”. To our knowledge no study of the waters of this watercourse had been made before. This study involved two stations, one upstream and the other downstream of the Soueir wadi, doing a bi-monthly frequency of water samples and benthic macro invertebrates. The physicochemical characterization concerned nineteen (19) physicochemical parameters (temperature, pH, conductivity, salinity, sodium, potassium, calcium, magnesium, chlorides, sulphates, bicarbonates, orthophosphates, nitrates, nitrites, ammonium, biological oxygen demand, chemical oxygen demand, suspended solids and dissolved oxygen). The biological quality assessment is based on the calculation of the Shannon - Weaver (H ') diversity index, the equitability index (E), the Simpson index (D) and the Belgian Biotic Index (BBI). The results show that the values of the measured different physicochemical parameters are below the accepted standards with the exception of nitrates which exceed 50 mg / l in some periods of the year. Similarly, the Shannon - Weaver diversity index measure shows relatively low upstream and greater diversity downstream of the stream. The equitability indices E and D reveal an almost absolute dominance of a single group of animals: the gastropods. However, the values of the Belgian Biotic Index (BBI) indicate a fairly good water quality. These results can be explained by the combined effects of intense agricultural activity in the Drader - Soueir Basin and variations in the flow of water in the watercourse, as well as the nature of the habitat.