Technical Abstract: Intraspecific competition is the basis for biological control of aflatoxins, but there is little understanding of the mechanism(s) by which competing strains inhibit toxin production. Evidence is presented which demonstrates a relationship between strength of the vegetative compatibility reaction and aflatoxin production in A. flavus and A. parasiticus using the suspended disc culture method. Combining aflatoxin-producing isolates belonging to different vegetative compatibility groups (VCGs) resulted in a substantial reduction in aflatoxin yield. Pairs of aflatoxin-producing isolates within the same VCG, but showing weak compatibility reactions using complementary nitrate-nonutilizing mutants, also were associated with reduced levels of aflatoxin B1. In contrast, pairings of isolates displaying a strong compatibility reaction typically produced high levels of aflatoxins. These results suggest that interactions between vegetatively compatible wild-type isolates of A. flavus or A. parasiticus are cooperative and result in more aflatoxin B1 than pairings between isolates that are incompatible. Successful hyphal fusions among spore germlings produce a common mycelial network with a larger resource base to support aflatoxin biosynthesis. By comparison, vegetative incompatibility reactions result in the death of those heterokaryotic cells composed of incompatible nuclei and may disrupt the formation of mycelial networks at the expense of aflatoxin biosynthesis.