This study aims to evaluate the performance of the sewage treatment plant of the lagunage type with activated sludge located in the town of Skhirat of the area Rabat-Sale-Kenitra in the vicinity of the Atlantic Ocean. Sampling was carried out monthly during the year 2013 follow- up by specific physicochemical analyses in terms of the chemical demand for oxygen (DCO), the biological request oxygenates of them (DBO5) and suspended matter measures it (SM). These parameters are measured at the laboratory by the device DCOmétre, DBOmétre as well as SM by a cellulose membrane filter. On the level of this sewage treatment plant, the output acquired in terms of DCO, DBO5 and SM is respectively of annual percentage average of about 87,26%, 88,35% and 68,37% what is very significant on the level downstream of this STEP and conforms to the national standards and international ; this is confirmed by the Test t unilateral for two matched samples represented by one p-value 0.0001 significant for the difference between the averages of entry and exitof the STEP. As well as the ratios of DCO/DBO5 and DBO5/DCO, indicate to us that the biodegrad ability is not difficult, represented respectively by values of about 2,05 and 0,49 what means what means that thisstation uses a kind of satisfactory biological treatment.