The aim of this study was to isolate and identify algal-lytic bacterium that tends to kill the harmful Microcystis aeruginosa and to determine the algal-lytic activity of the bacterial culture filtrate. In the screening of algal-lytic bacteria, we isolated a bacterium which had potent algal-lytic effects on fresh water algal blooms. Among of algal-lytic bacteria isolated in this study, isolate ... [Show full abstract] JS-9 was the strongest algal-lytic activity against M. aeruginosa. Isolate JS-9 was identified on the basis of morphological and biochemical characteristics. Thus, this isolate was designated as Pseudomonas sp. JS-9. The optimal culture conditions were 25°C, initial pH 7.0, and 1.0% (w/v) NaCl concentration. Pseudomonas sp. JS-9 showed algal-lytic activity through indirect attack, which excreted active substance into the culture filtrate. When 10% culture filtrate of JS-9 was applied to M. aeruginosa, 90% of algal cells were destroyed within 3 days. In addition, the algal-lytic activities were increased in dose and time dependent manners. Taken together, our results imply that Pseudomonas sp. JS-9 and its secreted compounds could play an important role in regulating the onset and development of algal blooms in the natural fresh water environments.