The availability of a new resuspension medium and the isolation of a strain of Pseudomonas aeruginosa unable to use quinic acid for growth have facilitated the evaluation of shikimic and quinic acids as precursors of pyocyanine. Shikimic acid or its derivatives were shown to account for virtually 100% of the carbon atoms of the phenazine nucleus of pyocyanine. The portion of the aromatic pathway where the branch point leading to pigment synthesis appears to occur has been indicated by competition experiments.