This work aims to study the seasonal fluctuation in physicochemical characteristics, trophic status, and some chemical and biological pollutants influencing phytoplankton diversity, and water quality in raw and treated water at Kafr El-Shinawy drinking-water treatment plant, Damietta - Egypt seasonally during 2018. Phytoplankton distribution was affected by the trophic status of water, level of pollutants, and physicochemical treatment processes (coagulation, flocculation and sedimentation) of water. The predominance of phytoplankton species, especially Aphanizomenon flos aquae (Cyanophyta), Gomphosphaeria lacustris (Cyanophyta), Microcystis aeruginosa (Cyanophyta), Nostoc punctiforme (Cyanophyta), Oscillatoria limnetica (Cyanophyta), Pediastrum simplex (Chlorophyta), and Melosira granulata (Bacillariophyta) in treated water was much less than in raw water. Trihalomethanes (THMs) levels in treated waters were higher than in raw water. On the contrary, lower concentrations of heavy metals were recorded in treated water. Phytoplankton cells exhibited a bioaccumulation capacity for heavy metals especially Mn, Zn, and Fe in both raw and treated water. Intracellular levels of microcystins were lower (0.003 - 0.011µg L−1) whereas the extracellular levels were higher (1.00 – 2.01 µg L−1) in treated water than raw water, and the former recorded the highest level in raw water during summer (1.3 µg L−1). Hence, the levels of  dissolved microcystins and THMs in treated water were higher than the allowable limit, especially during summer, the season of luxurious growth of Microcystis species.