The acidic heteropolysaccharide released by a water-bloom cyanobacterium Microcystis aeruginosa FACHB-912, which contained 8.3% protein, was used as the carbon source in stable enrichment cultures inoculated with natural aquatic microbial communities. The process of microbial breakdown of the exopolysaccharide and composition of the microbial community related to degrading exopolysaccharide were investigated. After inoculation of aquatic microbial community from Lake Taihu, enrichment cultures were obtained. In the enrichment culture, the exopolysaccharide was degraded immediately. The breakdown of the exopolysaccharide slowed markedly after about 18d, left a nondegraded fraction after 37d. The above data, together with comparison experiments on degradation capability of bacterial communities from various aquatic environments, indicated that the exopolysaccharide from M. Aeruginosa FACHB-912 could indeed be degraded by microorganisms coexisting only with the water bloom in nature. Denaturing gradient gel electrophoresis (DGGE) analysis showed composition of the microbial community did not change significantly during exopolysaccharide degradation. Phylogenetic analysis on DNA fragments, which were excised from DGGE gels, placed three degraders in Sphingomonas, one in Phaeospirillum, one in Pseudomonas, one in Rhodocyclaceae, one in Hylemonella, and one in Mycobacterium.