Rifamycin Z is a novel ansamycin produced by a mutant of Nocardia mediterranea. Physico-chemical data indicate that it possesses a lactone-type structure directly derived from rifamycin W. During our studies on mutant strains of the rifamycin producer Nocardia mediterranea, we isolated a morphological variant which produced a mixture of novel ansamycins. We report the structure elucidation of the major component, rifamycin Z, together with its proposed biogenetic relationship with the ansamycins so far reported.