Rifamycins, which was recently discovered by Sensi et al. in the culture broth of a Streptomyces sp., has been obtained from the culture broth of a Nocardia sp., C-521 strain. The taxonomy and fermentation of the new Nocardia sp. were described. To isolate the antibiotic complex,  the broth was extracted with petroleum ether. The petroleum ether extract was further purified by silica gel chromatography to give C-521 substance A, which was proved to be identical with rifamycin O. By distribution of the petroleum ether-insoluble residue between ethyl acetate and phosphate buffer solution (pH 7.0-7.2),  a crude product of C-521substance B was obtained from the latter. This was presumed to be rifamycin B from the chemical behavior. Evaporation of the ethyl acetate layer followed by further purification through countercurrent distribution, afforded another new antibiotic (C-521substance C). Antibactrial spectrum of the C-521substance A (rifamycin O) was presented.