Under the effect of streptomycin and hygromycin B there were obtained morphological mutants of Str. hygroscopicus differing in their capacity for synthesis of the antibiotic and a complex of proteolytic enzymes, as well as their resistance to high concentrations of the antibiotics. The mutation of resistance to the aminoglycosides with preservation of the initial phenotype did not result in significant changes in biosynthesis of hygromycin B and the enzyme. At the same time the antibiotic resistant mutants with changed morphology of the colonies were characterized by different capacity for the biosynthesis of the enzyme and antibiotic. Mutants with high, productivity of hygrolytin, not producing hygromycin B were most valuable. Production of hygromycin B correlated with the culture capacity to form aerial mycelium and spores. On the contrary, synthesis of hygrolytin by Str. hygroscopicus did not depend on the morphological characteristics of the culture: strains active with respect to hygrolytin synthesis were found among both the mutants forming abundant aerial mycelium and those not forming it. The study on the biochemical characteristics of the cultures showed that by the lipid content and fatty acid composition they belonged to the same group.