The monthly variations of phytoplankton composition as well as physico-chemical characteristic of irrigation and drainage canals water in Western Damietta were recorded over the year 1998, from January to December. The presence of agrochemicals has differential effects on water quality and phytoplankton population. There  were 126 species of algae identified at the irrigation canal, Omm Dongol (81 Bacillariophyta, 14 Euglenophyta, 13 Chlorophyta, II Cyanophyta, 6 Dinophyta, and I Cryptophyta). The number of species at the three drainage canals, El Senania, Marssok and Gamassa, was 28, 68 and 56 species, respectively. The species at El Senania, Marssok and Gamassa were respectively identified as 14, 41, 33 Bacillariophyta, 3, 10, 5 Cyanophyta, 7, 5, 6 Euglcnophyta, 3, 7, 8 Chlorophyta, 1, 4, 2 Dinophyta, 0, I, 1 Cryptophyta and 0, 0, 1 Chrysophyta. Phytoplankton standing crop would come from the contribution of Chlorophyta, Cyanophyta and Bacillariophyta representing 74, 23 and 2.5%, respectively in irrigation water and of Cyanophyta in drainage water. The number of species was dependent on Bacillariophyta. Total phytoplankton biomass resulted from the considerable growth of Euglenophyta, Chlorophyta and Bacillariophyta in irrigation canal and from Cyanophyta and Bacillariophyta at drainage canals. Diversity index indicated that pollution level for irrigation water was slight and heavy to moderate for drainage water. Moreover, saprobic index revealed the presence of B' -mesosaprobic in irrigation water and α to α" -mesosaprobic forms of algae in drain water. In addition, compound eutrophication indicated that the nature of irrigation water was mesotrophic while drain water was eutrophic.