The structure and dynamic of phytoplankton assemblages in Great Bitter Lakes were studied seasonally duringthe period from May 1999 to February 2000. Population density varied between112198X103 cells. l-1 in springand 2382X106 cells. l-1 in winter. In terms of cell number, Bacillariophyceae was the most important group(mean annual percentage contribution was 67%), which remained dominant throughout the study period.Cyanophyceae ranked the second abundant group (14 %) dominated in autumn and winter. The contribution ofDinophyceae and Chlorophyceae were 8.6 and 7.3% respectively. The other classes contributed with very lowshare of the total phytoplankton. The relationship between phytoplankton composition and physico-chemicalvariables was studied. CCA analysis indicated that water salinity, and temperature are the most important factorsaffecting phytoplankton distribution. The trophic status index indicated that the lake ranked betweenmesotrophic and moderately eutrophic. In winter, the lake was under blooming due to the luxuriance growth ofNitzschia pseudo delicatissima Hasle. and Phormidium sp. The impact of pollution was significantly effective inthe western side of the lake compared to the eastern one.