Gibberella fujikuroi(anamorphFusarium moniliforme) mating group A isolates from corn and sorghum were categorized into different vegetative compatibility groups, usingnitmutants (mutants unable to reduce nitrate or chlorate) to observe heterokaryosis among the compatible isolates. Vegetative incompatibility seems to be widespread in nature in this fungus and is controlled by nuclear genes calledhetorvicgenes. A total of ninehetgenes were estimated forG.fujikuroimating group A. Based on the independent segregation of ninehetgenes, 512 vegetative compatibility groups may be present in nature. However, only 13 vegetative compatibility groups were identified among conidial isolates recovered from Nebraska. The frequency of individual vegetative compatibility groups in a disease complex may be useful for monitoring pathogenicity genes in this fungus.