Red tides of a harmful dinoflagellate, Heterocapsa circularisquama, have caused mass mortality of bivalves such as oysters in Western Japan since 1988. For the purpose of microbial control of the red tide occurrence, H. circularisquama-killing microorganisms were screened. A marine bacterium EHK-1, which had a strong algicidal effect on H. circularisquama, was isolated from seawater of Etajima Bay, the Seto Inland Sea of Japan. EHK-1 was a gram-negative, long rod shaped bacterium that showed a gliding-like motility. Pylogenetic analysis of 16S rRNA gene sequences indicated EHK-1 was a novel marine bacterium belonging to proteobacteria gamma-subgroup. EHK-1 killed H. circularisquama within 24 hours when this bacterium was inoculated at the density of 1x104 cells/ml to H. circularisquama culture in the exponential phase. During the killing process, both of the motility loss and the lysis of the microalga were observed. The filtrate from the co-culture of the EHK-1 and H. circularisquama was lethal to H. circularisquama. It suggests that EHK-1 killed H. circularisquama by means of extracellular substances.