ABSTRACTSelection and identification of nitrate-nonutilizing (nit) mutants ofAspergillus flavuswas compared on six chlorate-containing agar media. Three isolates ofA. flavuswere used.Nitmutants isolated comprised three phenotypic classes, equivalent toniaD, nirA-, andcnxmutants ofA. nidulans.There were significant differences among isolates and among media in number and types ofnitmutants isolated. The most effective medium contained only nitrate as a nitrogen source, whereas most previously described media for isolation ofnitmutants contain both nitrate and an amino acid. Morphology ofnitmutants was more distinctive on this medium than on other media. The nitrate-only medium yielded mainlyniaDmutants; other media yielded morenirA-mutants, which complementniaDmutants. Fewcnxmutants were found. Use of a nitrate-only medium in conjunction with another medium should optimize isolation ofnitmutants and complementary pairs of mutants.