The maximal production of aflatoxin B₁ and G₁ by A. flavus ATCC 15517 was reduced by 98% and 99% respectively, in the mixed culture with A. awamori var. fumeus in comparison with that in the monoculture of A. flavus. An important cause for the reduction in aflatoxin formation was found that A. awamori var. fumeus excreted aflatoxin degrading factor(s) into the medium during the growth which precipitated by the addition of ammonium sulfate (0∼80%).