DL-2,3,4,5,6-pentafluoromandelic acid (PFM) specifically inhibits the growth of Pseudomonas putida (ATCC 12633) on medium containing mandelate as sole carbon and energy source by competitive inhibition of mandelate dehydrogenase. PFM is not metabolized and is neither an inducer of the mandelate catabolic enzymes nor an antagonist of induction. Mutants resistant to the inhibitory effects of PFM (PFMr) were isolated; most prove to be superinducible, i.e. synthesize corrdinately the mandelate-specific catabolic enzymes at elevated levels following induction. In at least one case the PFMr mutation maps very near the structural genes that encode the enzymes functional in the first two steps of mandelate catabolism. It is reasoned that the PFMr mutation is of the promotor type. Resistance to substrate analogs such as PFM offers a general method for isolation of regulatory mutants in catabolic metabolism.