The present work was carried out at a tertiary sewage water treatment plant located at El- kattameya city, Cairo, Egypt, for 12 months, January- December 2004. The phytoplankton standing crop decreased from 228.4 cells x10⁵/L at the oxidation tank during summer to 3.7cells x10⁵/L at the effluent tank during autumn. Phytoplankton community was represented by 88 species belonging to 42 genera and 7 classes namely; Cyanophyceae  forming 68% of total phytoplankton standing crop, Chlorophyceae (16.4 %), Bacillariphyceae (13.7 %), Euglenophceae (1.5 %), on the other hand Xanthopyceae, Cryptophyceae and Dinophyceae were sporadic and poorly represented. The leading phytoplankon species were Phormidium molle, Chamydomonas snowii, Nitzschia pellucida and Euglena gracilis. All these species regarded as eutrophic indicators. The tanks under investigation, especially the oxidation tanks are considered to be eutrophic, with decreasing level of eutrophication towards effluent tank, indicate the improvement of water quality. Bacteriological counts using total fecal and fecal coliform were also determined during winter and summer seasons. Total coliform and fecal coliform densities were much higher at the collector tank than at effluent during both winter and summer seasons. Seasonal variations in chlorophyll a at the four tanks of the sewage water treatment system were 190.5 mg/L recorded at oxidation tank during summer season, while the lowest content of 7.3 mg/L was recorded at effluent tank during autumn season. The trophic state index was calculated from chlorophyll a values, the highest value (82.1) was observed at oxidation tank during summer and the lowest one (50.0) was obtained at effluent tank during autumn.