Bio-Floc consists of phytoplankton,bacteria,aggregates of living and dead particulate organic matter,and grazers of the bacteria.It not only can regulate water quality,but also can supply the food protein for breeding animals.For the study of bacterial community structure of BioFloc in freshwater culture systems,the key technologies of Israeli Avnimelech team about Bio-Floc in the field of aquaculture were used in this paper.Glucose was added as the carbon source which makes a C∶N ratio of 20∶1 be maintained during the experiment.The bacterial communities of BioFloc were analyzed by using the PCR-DGGE(PCR-denaturing gradient gel electrophoresis)on the 5th,10th,and 15th day,respectively.The results indicated that the similarity of bacterial communities was highest on the 5th day and 10th day which was 67.4%,and was lowest on the 5th day and 15th day which was 40.5%.Diversity of bacterial species in the Bio-Floc was maximal on the 10th day and minimal on the 15 day.The acquired sequences of 24 bands in DGGE gel were performed by BLAST searches against NCBI database,and the sequences including the closely related sequences were aligned with Clustal W program in BioEdit software for phylogenetic analysis.The main microbes represented by 24 main bands in DGGE gel were Alphaproteobacteria,Gammaproteobacteria,Betaproteobacteria,Actinobacteria,Bacilli and Bacteroidetes.Among these bacteria,the α-,β-,and γ-proteobacteria are the major bacterial groups,but Actinobacteria bacteria exist on the 10th day and the 15th day.α-proteobacteria was the dominant bacteria during the entire process.The specific bacteria at different stages were usually the flocculant-producing bacteria:Acidovorax,Aeromonas,Agrobacterium only for 5th day;Bacillus,Rhodococcus for 10th day and 15th day,respectively.This study first found that the composition and diversity of bacteria of the Bio-Floc used in freshwater aquaculture systems are extremely rich.The study of functional microbial composition of the Bio-Floc may help to lay the foundation for the further application of the bio-floc technology in actual aquaculture production.