Microcystis spp. produces large amounts of exopolysaccharides(EPSs) including capsular polysaccharides(CPSs) which form mucilaginous matrix around the cells, and water-soluble released polysaccharides(RPSs). To understand their ecophysiological roles in cyanobacterial bloom, we urgently need more information on chemical characters of EPS. In this paper, colonial Microcystis strains were isolated from Lake Taihu. After incubation for about 18 months in the lab, some strains turned to be unicellular form. The comparative study of chemical characteristics of EPSs from five colonial strains and four unicellular strains showed that(1) all the EPSs were acidic heteropolysaccharides containing the deoxyhexoses;(2) the uronic acid content was lower in CPS(1.2%-2.1%) than in RPS(2.4%-6.2%) for all Microcystis strains;(3) acetate and sulfate groups were present in all EPSs, of which the acetate content was higher in CPS than in RPS for each strain, and higher in CPSs(4.1%-6.6%) of all colonial strains than in CPSs(2.0%-3.2%) of all unicellular strains. The chemical characteristics of EPS were discussed with respects in the solubility of EPS in water, the roles in colony formation and the ecological roles in the aquatic environments. It was thought that among these chemical characteristics, acetyl substitution might be an important factor affecting the ecophysiological roles of EPS in cyanobacterial bloom.