Co-cultivation of a strain of Aspergillus parasiticus, capable of making aflatoxins, with blocked mutant strains, capable of producing none or only a low level of aflatoxins, reduced the net yield of aflatoxins more than that expected based on spore recovery. Yields of aflatoxins were 8-fold less for a norsolorinic acid-producing strain, 14-fold less for an averantin-producing strain, 6-fold less for an averufin-producing strain, and 21-fold less for a versicolorin A-producing strain when co-cultured in equal amounts with a wild-type strain of Aspergillus parasiticus. Even when the wild-type strain was initially present in 100-fold excess, with two of the mutant strains, reduced aflatoxin production was still observed.