Abstract: The seasanal changes in the ecological features of phytoplankton community including species composition,population size,dominant species,saprobic indicators,diversity index, diatom biotic index,comprehensive index ect,were preliminarily studied at 30 sampling stations in the Dalaihu Lake,Manzhouli.By using population densities(individuals and cells per litre),dominant species,saprobic indicators,diatom biotic indices and compre hensive index as the important biological parameters,the trophic level and saprobic degree were evaluated for both the whole and subareas of the lake.In the Dalaihu Lake,the annual mean density of phytoplankton reached 54.7?06 cells per litre or 2.3?06 individuals per litre. The diatom biotic index was 149.3,the comprehensive index was 5.6 and the saprobic indicators comprised 65% of the total species in the community.In Spring,the dominant species of planktonic algae were almost entirely composed of green algae,especially Crucigenia quadrata and Oocystis.In Summer,Autumn and Winter,the dominant species of planktonic algae were almost entirely composed of blue-green algae,especially Microcystis(M.aeruginosa and M. flosaquae)and Anabaena(A.spiroides and A.flos-aquae). All these features indicate that the lake has suffered from moderate pollution and can be regarded as a blue-green and green algae-eutrophic lake.