In our screening program for antibioticproducing organisms, we found that Streptomyces strain No. 768 isolated from a Philippine soil by SHINOBU (unpublished) produced a pink or reddish orange pigment which showed antimicrobial activity against Gram-positive bacteria. The pigment was isolated as dark red needle crystals, determined to be a new flavin compound and named "Roseoflavin". Streptomyces strain No. 768 was cultured at 30°C for 80--120 hours in a starch, soybean meal, meat extract and salt medium; the pigment was isolated via adsorption on diatomaceous earth, elution with hot 5% pyridine, concentration, cooling and cellulose column chromatography purification. Roseoflavin showed antimicrobial activity against Gram-positive bacteria (with variable MICs between agar and broth dilution methods) and had LD50 values of 400 mg/kg (intraperitoneal injection) and > 3,000 mg/kg (oral administration) in mice. Its molecular formula was C18H23N5O6 (from elemental analysis, mass spectrum and PMR spectrum of the acetylated compound). X-ray crystallographic analysis of a derivative and comparison with synthesized D-isomers demonstrated that roseoflavin is 7-methyl-8-dimethylamino-10-D-ribitylisoalloxazine.