Estuaries are the contact areas between fresh and sea waters. These areas are very unstable ecosystems, and the hydrological. physicochemical parameters are extremely variable. Phytoplankton is a very important part of ecosystem and is extremely susceptible to environmental variations. Throughout this analysis we propose to assess a distribution of phytoplankton groups in an estuarine zone (the mouth of Oued Mazafran). and to establish distributions along the adjacent coast as a function of time and space concerned. Hydrological and phytoplankton samples were taken in 04 stations which were located in the region of (Oued Mazafran) from the period (17 January to 18 April) of 2018. The results of salinity, temperature, abundance, diatoms, diatom index and specific richness allow us to say: Stations 1 and 2: (Colonel Abbas beach and river mouth) we have the river's transition ecosystem with a high salinity variations from 16.32 to 30.33 at Station 2. and from 30.08 to 36.86 at Station 1. We note the presence of euryhaline species. Station 3: A freshwater environment (Mazafran Oued) salinities of 0.8 with a large temperature range of 12 to 18 °C and low oxygen concentration of 1.33 mg.dm-3 we have Phytoplankton species of freshwater.Stations 4: Seawater ecosystem with little variation (shellfish center). During this time the specific wealth reported at all stations is 343 species; we have 68.8 per cent of diatom. Station 3 (Oued Mazafran) registered its highest cell density of 11370 cell.dm-3.