A three-year study was conducted to evaluate the use of a nonaflatoxin-producing strain of Aspergillus parasiticus (NRRL 13539) as a biocompetitive agent for the control of preharvest aflatoxin contamination of peanuts. The agent was added to the soil of the environmental control plot facility at the National Peanut Research Laboratory and tested by subjecting peanuts to optimal conditions for the development of aflatoxin contamination. Edible peanuts from the treated soil contained aflatoxin concentrations of 11, 1, and 40 ppb for crop years 1987, 1988, and 1989, respectively, compared to untreated peanuts with 531, 96, and 241 ppb, respectively. In addition, treatment in 1989 with low and high inoculum levels of a UV-induced mutant from the NRRL 13539 strain resulted in aflatoxin concentrations of 29 and 17 ppb, respectively, in edible peanuts. Soil populations of the biocompetitive agents were not higher than populations of wild strains of A. flavus/parasiticus in untreated soil subjected to late-season drought stress. This is an important ecological consideration relative to the utilization of this biocontrol system.