The changes of primary producer, zooplankton, and characteristics of bacterial community were compared with the bloom of Microcystis aeruginosa in the Chusori, Daechung Reservoir where conventional summer bloom was observed. Primary producer changed from diatoms at spring to M. aeruginosa with the concentration of 4.6x10^8 cells 1^-1 at summer. Zooplankton changed into large-sized copepod. Difference of the total bacterial number between pre-bloom and blooming period was not large. The saprophytic number was average of 1.27×10^4CFUS ml^-1 that was 0.12% of the total bacterial number. Bacterial production rate was 3.95㎍C1^-1hr^-1 before July and 2.77㎍gC 1^-1hr^-1 during blooming period. Characteristics of bacterial community showed bacteria did not readily use increased organic compound.