A p-fluorophenylalanine-resistant mutant (accphe) which grows on minimal medium has an altered prephenate dehydrogenase and maps at the try-1 locus. Two other tyr-1 mutants which require tyrosine for normal growth can eventually grow on minimal or minimal plus p-fluorophenylalanine (FPA). The three different tyr-1 mutants all accumulate phenylalanine when incubated in minimal medium. FPA is incorporated into protein at only 10-15% the wild-type rate when mutant conidia are incubated in a minimal salts-glucose system. Under the same conditions, phenylalanine incorporation in the mutants is initially the same as in wild type. When tyrosine is included in the medium, resistance to FPA is lost, phenylalanine accumulation is prevented, and FPA is incorporated into protein at the wild-type rate. Tyrosine apparently prevents the overproduction of phenylalanine by preventing the overproduction of chorismate and prephenate.