An evaluation was made of the genotypic diversity (DNA fingerprinting) of 269 A. flavus strains, including subpopulations isolated from grain sampled at harvest (91 unique ‘fingerprints’ or genotypes/128 strains), field soil (26 genotypes/31 strains), maize insects (49 genotypes/52 strains) and air-spora (56 genotypes/58 strains), from a maize field near Kilbourne, Illinois. Eight A. flavus genotypes were isolated from grain samples harvested in different years (1988–1991). Genotype 36, isolated from three maize samples, matched the DNA fingerprint of a K. E. Papa strain NRRL 19997, isolated from maize grown in Georgia. Ninety-eight percent of the A. flavus genotypes produced sclerotia and 53% produced aflatoxin. Contrasts of DNA fingerprints revealed two matches involving subpopulations from grain and soil, one match for grain and maize insects, and no matches for grain and air-spora. The high genotypic diversity recorded for each subpopulation, in addition to a limited sample size, precluded any assessment of the relative importance of these subpopulations as sources of A. flavus infective inoculum. Aspergillus parasiticus was routinely isolated from soil samples.