A bacterium which inhibited the growth of a green alga Chlorella sp. was isolated from upper-layer water of the brackish lake Nakanoumi and identified as Enterobacter aerogenes. The bacterium produced relatively stable algal growth inhibitors(AGI) which decreased dramatically the chlorophyl content of Chlorella in the initial growth stage under continuous illumination although the cell number was unchanged. The partially purified AGI also inhibited the growth of other green algae and diatoms as well as Chlorella. From the behavior in gel filtration (Sephadex G-100 and Sepharose 4B), paper partition chromatography, and ionexchange chromatography(DEAE Sephadex A-50), it was suggested that these AGI were high molecular substances containing saccharides as the constituent.