Technical Abstract: Aspergillus flavus Link is an economically important fungus which produces carcinogenic compounds known as aflatoxins in agricultural crops such as corn, peanuts, cotton and tree nuts. The fungus has no known sexual stage; consequently, most studies on its genetic variability have been evaluated mainly by characterizing isolates based on vegetative compatibility. Complementary nitrate-nonutilizing (nit) mutants are commonly used to identify compatible isolates. This study was conducted to determine for the first time the VCGs of A. flavus isolates from a California pistachio orchard. Sclerotium size, aflatoxin production and VCGs were analyzed in order to estimate the diversity of the sample population. A better understanding of the population structures of A. flavus will faciliate the development of effective biocontrol strategies.