Of several types of chemical surfactants and biosurfactants, only the culture broth of Bacillus subtilis C1 containing surfactin at 10 mg l(-1) completely inhibited the growth of Microcystis aeruginosa, a bloom-forming cyanobacterium in highly eutrophic lakes. The broth with 10 mg surfactin l(-1) also removed 85% of the maximally grown M. aeruginosa (chlorophyll-a concentration, 1000 mug l(-1)) within 2 d, and the removal efficiency was enhanced by Ca2+ over 1 mM. The growth of Anabaena affinis, another bloom-forming cyanobacterium, was also inhibited about 70% with surfactin at 10 mg l(-1) broth. However, the effect of the broth was delayed over 3 d in the green algae, Chlorella vulgaris and Scenedesmus sp., and was negligible in a diatom, Navicula sp., indicating the potential for the selective control of cyanobacterial blooms.