A bacterium 5N-3 possessing a remarkable inhibitory effect on the growth of Gymnodinium nagasakiense was isolated from Uranouchi Inlet, Kochi. This bacterium was tentatively identified as Flavobacterium sp. The growth inhibiting effect of 5N-3 on G. nagasakiense was drastic in particular when the alga was in the logarithmic growth phase, and cell density decreased to less than 1% of the initial concentration within 4 days after inoculating 5N-3, indicating that the effect was algicidal. The effect was obtained when the density of the bacterium was more than 10⁶ cells/ml. However, they grew very rapidly up to 10⁸ cells/ml by using extracellular released organic carbon from various phytoplankton species. On the other hand, the algicidal effect of 5N-3 was only observed on G. nagasakiense but not on Chattonella antiqua, Heterosigma akashiwo, or Skeletonema costatum. These results indicate that the effect of 5N-3 was G. nagasakiense-specific and suggest that it grows to a level of cell density effective in inhibiting the alga in the field by using naturally oc-curring organic carbon from phytoplankton.