The blue-green algaAnacystis nidulanswas treated with different concentrations of colchicine, sulfanilamide, hydroxylamine, and diethyl sulfate. Its growth was inhibited by relatively low concentrations of hydroxylamine and diethyl sulfate but not by those of colchicine. Colchicine stimulated the algal growth at a concentration of 1000 mg/l and inhibited it at a concentration of 5000 mg/l.Strains of the alga that are resistant to high concentrations of sulfanilamide were produced by subculturing it in increasing concentrations of the drug. These were found to be fully cross-resistant to the same concentrations of sulfaguanidine but not to sulfathiazole. Repeated subculture of the alga in an hydroxylamine medium failed to increase materially its resistance to hydroxylamine.Treatment ofAnacystis nidulanswith the mutagen diethyl sulfate failed to induce any streptomycin-resistant mutants.