The changes of taxa within the particle-attached bacterial assemblage during the decomposition of Microcystis blooms were investigated under darkness and anoxic condition in mesocosm experiments. During 14 days of darkness incubation， chlorophyll-a (Ch-a) concentration decreased from 2000 μg/L to 5 μg/L. Samples were collected on days 0， 2 and 14 for bacterial 16S rRNA analysis， based on rapid decreases in the Chl-a concentration of water column. The total bacterial community DNA was extracted and 16S rRNA genes were amplified by polymerase chain reaction， cloned and sequenced of selected samples. The results showed that the abundance of attached bacteria increased significantly， and the composition of the particle-attached bacterial communities varied temporally during the decomposition of Microcystis blooms. The bacterial assemblage appeared to be dominated by members of Bacteroidetes， Alphaproteobacteria and Betaproteobacteria. Shift of some genera of Alphaproteobacteria and Alphaproteobacteria was also observed. Additionally， we found that the family Sphingomonas， affiliated with Alphaproteobacteria， identified as a microcystin-degrading bacterium， dominated the particle-attached bacterial communities. The results from the present study， together with previously published data highlighted the need for more studies concerning the bacterial degradation process in order to trace the environmental fate of microcystins in field conditions.