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In an effort to identify a bio-agent capable of controlling cyanobacterial blooms,we isolated a bacterial strain,A27,which exhibited strong algicidal activity against the dominant bloom-forming species of Microcystis aeruginosa in Lake Taihu.Based on 16S rRNA gene sequence analysis,this strain belongs to the genus Exiguobacterium.This is the first report of an algicidal bacterial strain belonging to the genus Exiguobacterium.Strain A27 exhibited algicidal activity against a broad range of cyanobacteria,but elicited little or no algicidal activity against the two green algal strains tested.The algicidal activity of strain A27 was shown to be dependent on the density of the bacteria and to have a threshold density of 1.5×106 CFU/mL.Our data also showed that the algicidal activity of strain A27 depended on different growth stages of Microcystis aeruginosa (exponential ≈ lag phase ＞ early stationary) rather than that of the bacterium itself.Our results also suggested the algicidal activity of strain A27 occurred via the production of extracellular algicidal compounds.Investigation of the algicidal compounds revealed that there were at least two different algicidal compounds produced by strain A27.These results indicated that strain A27 has great potential for use in the control of outbreaks of cyanobacterial blooms in Lake Taihu.