The mutant under study, designated A10, is derived from a Nocardia mediterranei strain, N813, which is a high rifamycin B producer. A10 is auxotrophic for aromatic amino acids but unlike A8 (see preceding paper) produces the same amount of rifamycin B as the parent. Shikimic acid and 3-dehydroshikimic acid are accumulated in the fermentation broth of this mutant. It was shown to be blocked in one of the enzymes leading from shikimate to chorismate. No formation of shikimate-3-phosphate from shikimate and ATP could be detected in vitro using crude extracts of this mutant and of the parent. As mutant A10 is only defective in the biosynthesis of aromatic amino acids and not in the biosynthesis of rifamycins it would appear that the seven-carbon amino unit of the rifamycin-chromophore must be derived from an intermediate of the shikimate pathway not behind shikimate. By referring to the results of the preceding paper it can be seen that the origin of this moiety can definitely be localized between 3-deoxy-D-arabinoheptulosonic acid-7-phosphate and shikimate.