In a previous investigation antibiotic preparations effective against fungi were obtained from cultures of an unidentified species of Streptomyces. Because the properties of these preparations appeared unique, a name, antimycin, was proposed for the active material. Continued investigation has now resulted in the isolation of an active crystalline substance which is described in this report. The name "antimycin A" is proposed for this crystalline substance, since there is evidence of additional antibiotic factors in the crude preparations. The isolation was accomplished by means of solvent fractionation procedures, the active material being taken successively into alcohol, ether, and petroleum ether. Residues from the latter solvent were readily induced to crystallize. Antimycin A is an optically active, nitrogenous phenol with the probable molecular formula C28H40O9N2. Its properties appear to differentiate it from other previously known antibiotics. It is an extremely potent fungicide, producing inhibitory effects against Nigrospora sphaerica, for example, at dilutions as high as 1 : 100,000,000.