To control the growth of eurocaryotic and blue-green algae in anaerobic reactors of photosynthetically grown Rhodospirillaceae, the effect of algae inhibitors with different modes of action was examined. Tests were performed with mixed populations of green algae and blue-green algae, besides strains of the purple nonsulphur bacteriaRhodopseudomonas capsulata, Rhodospirillum rubrum andRhodomicrobium vannielii. Chloroxuron, a urea-derivative, was found to inhibit completely growth of green and blue-green algae at 5 ppm. When it was applied to the Rhodospirillaceae cultures, growth was not reduced and nitrogenase activity was not inhibited.