Phytoplankton was assessed quantitatively and qualitatively in regard to their abundance in the northern part of El-Umum drain, west of Alexandria at four stations to study the spatial, temporal, dominance and abundance of the phytoplankton community. The samples were collected monthly from April 2003 to April 2004. Atotal of 152 phytoplankton taxa were identified belonging to Bacillariophyceae (60), Chlorophyceae (46), Cyanophyceae (20), Euglenophyceae (17) and Dinophyceae (9). A limited number of these were recorded throughout the year, and showed an indication of organic pollution. The total densities (333.8x10(3) units l(-1)) were mainly a reflection of the trends in counts of Bacillariophyceae and Chlorophyceae (138.1 x 10(3) and 131.6 x 10(3) units l(-1), respectively). Monthly differences in the quantitative (18 x 10(3) to 1645 x 10(3) units l(-1)) and qualitative composition (41 to 113 taxa) of the phytoplankton communities in the different stations were marked, with recurrent high abundance in spring (910.4 x 10(3) units l(-1)), whereas lowest densities occurred in autumn (99.7x10(3) units l(-1)). Cyanophyceae was most abundant in July (67.5 x 10(3) units l(-1)), whereas Euglenophyceae in December (32.0x10(3) units l(-1)). Species diversity (H') gave a signal of the area is light to moderate polluted. The present study emphasized the need to use phytoplankton community as index of water quality.