Six mutants resistant to p-fluorophenylalanine (FPA) were selected on a medium containing aspartate as the sole source of nitrogen using a phenylalanine-requiring (phenA)auxotroph of A. nidulans as the wild type. The mutants, on the basis of genetic characterization, were found to be alleilic and located on the left arm of the linkage group III, approximately 13 map unit left to meth H locus, henceforth assigned to the symbol fpaV. At a fixed concentration of phenylalanine (23 micrograms/ml), the LD50 value of FPA for all the six mutants was found to be about three times more than that for the wild type strain. Affinity chromatographic purification of the enzyme phenylalanyl-tRNA (Phe-tRNA) synthetase from the mutant as well as the wild type strains, revealed that the wild type enzyme had about 1.4-fold higher affinity for phenylalanine as compared to that for FPA, both in the affinity column and in the catalytic reaction. However, the mutant enzyme showed almost a similar affinity for both in columns but a greatly reduced affinity for FPA in the catalytic reaction.