A new polyene antibiotic was isolated from the culture broth of a new Streptomyces, designated as Streptomyces noursei Hazen et Brown, 1950 var. jenensis nov. var. JA 3789. This antibiotic is a tetraene and was named tetramycin. The strain producing this antibiotic was isolated from a sample of humus containing loam collected in Vietnam. The morphological and physiological characteristics of Streptomyces JA 3789, the medium for antibiotic production, and the isolation, purification, physiochemical, and biological properties of tetramycin are described. Tetramycin is active against yeasts and fungi and was differentiated from related antibiotics by physical and chemical data.