The ability of two non-aflatoxin producing strains of Aspergillus flavus to interfere with aflatoxin production by a toxigenic A. flavus strain was examined using a replacement series with suspended disc culture method. Individual glass fiber discs, affixed to a pin suspended from the caps of scintillation vials, were inoculated with medium containing A. flavus conidial mixtures in different proportions (aflatoxin producer[ratio ]non-producer=100[ratio ]0, 80[ratio ]20, 60[ratio ]40, 20[ratio ]80 and 0[ratio ]100 by vol) at a constant total density (1×105 spores ml−1). Reductions in the total conidial density of these strains when grown alone, had little effect on fungal growth (mycelium dry weight) or aflatoxin production. Significant (P<0.0001) reductions in aflatoxin B1 were recorded when non-toxigenic strains represented any proportion of the inoculum mixture. Aflatoxin yield values were less than (P<0.0001) expected from the input ratios for toxigenic vs. non-toxigenic conidial inoculum within the replacement series. Aflatoxin yields were also reduced (P<0.001), with a corresponding increase in fungal growth (P<0.001), when conidia from aflatoxin producing strains were mixed in equal proportions. This suggests that the substantial inhibition of aflatoxin yield for inoculum mixtures results from the failure of spore germlings to establish a cooperative mycelial network.