Field and the laboratory studies were carried out to ascertain the potential impact of algicidal bacteria on Microcyctis aeruginosa from June 2005 to March 2007 in Beira Lake, Sri Lanka. M. aeruginosa, and M. wesenbergii quantitatively dominated in most sampling dates and constituted>75% of the phytoplankton cell densities when the bloom reached to its peaks. Densities of algicidal bacteria were relatively high with large fluctuations between 2.3x102 PFU ml-1 to 0.3x102 PFU ml-1. Three algicidal bacteria species were isolated from the lake and identified as Alcaligenes denitrificans, A. xyosoxydans, and Flavobacterium marinotypicum. The algicidal effect of the bacterium was studied and the results suggest that the bacteria did not release extracellular products inhibitory to M. aeruginosa, and that the bacteria killed the algae by direct contact. In the field, rapid decline of Microcystis bloom was detected when algicidal bacteria were increased. In the laboratory, when the bacterium were inoculated at low densities (104 cells ml-1) together with Microcystis, the bacteriumproliferated to 107 cells ml-1 and caused Microcystis cell lysis. Thus, the result of the present study strongly suggests that algicidal bacteria degrade M. aeruginosa bloom in natural freshwater environments.