An aquatic actinomycete capable of eliminating the brown tide causing marine alga Phaeocystis globosa was isolated from the surface sea water and the isolate named JS01 was characterized as Streptomyces on the basis of its 16S rRNA gene sequence. The supernatant of JS01 could lyse algal cells, implying that JS01 produced a latent alga-lytic compound. Considering this algicidal activity and the response of the algal cells, Chlorophyll a fluorescence decreased significantly in P. globosa in response to the JS01 supernatant when analyzed with flow cytometry. The algal cells experienced cell shrinkage and plasmolysis before disintegration after 72 h of treatment. The released algicide(s) were heat-tolerant, except above 121 A degrees C, and fluctuation in pH variations; even so, algicidal activity was also over 60 %. The maximum toxicity of JS01 was on the seventh day of culture, and the relative luminosity was 0.49 at that time when detected by luminous bacteria Vibrio fischeri. These results indicated that the Streptomyces sp. JS01 could function as a potential controller of Phaeocystis globosa blooms.