Streptomyces sp. No. B-5050 was found to produce a new antibiotic maridomycin. The taxonomic study on strain No. B-5050 was carried out and this strain was found to be a strain of Streptomyces hygroscopicus (Jensen, 1931) Waksman et Henrici, 1948. The cultural condition for the production of maridomycin was also investigated. In the course of our screening program for new antibiotics active against Gram-positive bacteria and mycoplasma, Streptomyces strain No. B-5050 was found to produce an antibiotic showing crossresistance against macrolide antibiotics. Strain No. B-5050 was found to belong to the Streptomyces hygroscopicus group and the antibiotic produced by the strain was assumed to be a new macrolide antibiotic. As a result of the isolation and characterization of this antibiotic from the culture fluid, it was found to be a new member of the macrolide antibiotics showing no characteristic ultraviolet absorption and was named maridomycin. Strain No. B-5050 differs from the typical strain of Streptomyces hygroscopicus (Jensen, 1931) Waksman et Henrici, 1948 in some respects, but in a broad concept, it was regarded as belonging to this species. The present paper describes the taxonomic characteristics, antibiotic activity, and fermentation of strain No. B-5050.