Abstract: The seasonal ecological features of phytoplankton community including its species composition, population size, dominant species, saprobic indicator, diversity index and diatom biotic index were preliminarily studied at 22 sampling stations in the Chaohu Lake, Anhui Province. By using population densities (individuals and cells per litre), dominant species, saprobic indicators and diatom biotic indices as the important biological parameters for monitoring and evaluating pollution conditions of water quality, the trophic level and saprobic degree of the lake were evaluated in whole and in subareas respectively.Dealing with the Chaohu Lake as a whole, the annual mean density of phytoplankton reached 14.7 ?106 per litre for individuals or 111 ?106 per litre for cells. The diatom biotic index was 96.7 and the saprobic indicators comprised 64.2% of total species in the community. Except in winter when Cyclotella comta had higher frequency and greater abundance the dominant species of planktonic algae were almost entirely consisting of blue-green algae especially Microcystis (M. aeruginosa and M. flos-aquae) and Anabaena. Their frequency attained 98.8%, and abundance, 79.7%, when all seasons were taken into account. In suumer and autumn, the frequency was up to 100%, and the abundance, 99.7%. All these indicate that the lake has suffered from moderate pollution and it might be regarded as a blue-green algae-eutrophic lake.