24 samples which were incoulated with Aspergillus flavus and Bcillus subtilis and cultured on the steamed soybean media under various conditions-pH, moisture and temperature were-investigated on the production of aflatoxin by the interaction of these two microorganisms. 1) The amount of aflatoxin produced by mixed cultures of Aspergillus flavus and Bacillus subtilis was decreased significantly rather than that of single cultures of Aspergillus flavus. 2) Maximum production of crude aflatoxin was 2,560 ppm $(B_1,;0.908;ppm;;B_2,;0.261;ppm;;G_1,;1.162;ppm;;G_2,;0.229;ppm)$ at 30% moisture, pH 5.0 and $20^{circ}C$, whereas minimum production was 1.107 ppm $(B_1,;0.341;ppm;;B_2,;0.104;ppm;;G_1,;532;ppm;;G_2,;0.130;ppm)$ at 63% moisture, pH 9.0 and $40^{circ}C$.