Aims: The study of an algicidal activity and mechanism of the isolated Pseudomonas fluorescens HYK0210-SK09 (SK09) against a winter bloomed harmful diatom, Stephanodiscus hantzschii. Methods and Results: SK09 was isolated from the Paldang reservoir, Korea and used to biological control of S. hantzschii. The inoculation of SK09 at the final density of 5 x 10(6) cells ml(-1) caused degradation of > 90% of S. hantzschii cells within 5 days. The algal cell lysis was achieved by a direct attack of the bacteria to the diatom cells, and the algicidal compound was located in the cytoplasm of the cell. As SK09 did not suppress Microcystis aeruginosa, Anabaena cylindrica, Coelastrum astroideum or Cyclotella meneghiniana, it appeared to attack S. hantzschii in a species-specific manner. Testing in an indoor mesocosms confirmed that SK09 effectively reduced S. hantzschii cells by 88% within 9 days. Conclusions: This bacterium is useful in regulating blooms of S. hantzschii. However, it should be studied in the future that their impact in shaping phytoplankton community and their activity in natural environments. Significance and Impact of the Study: The bacterium enabled us to develop a new strategy, to understand the interaction for anthropogenic control of harmful algal blooms in nature.