Two novel antimycin antibiotics, urauchimycins A and B, were isolated from a fermentation broth of a Streptomyces sp. Ni-80. The strain was isolated from an unidentified sponge. Their chemical structures were determined by 2D NMR analysis. They are the first antimycin antibiotics which possess a branched side chain moiety. They exhibited inhibitory activity against morphological differentiation of Candida albicans.