Yields of from 200 to 300 mg per liter of aflatoxins B1 and G, were produced by two strains of Aspergillusflavus in 20-liter fermentors under proper conditions of inoculum (well-dispersed growth) and aeration (0.5 volume per volume per min of air, 300 rev/min, 30 psi back pressure, baffles). Peak yields were usually attained in 72 hr, after which the aflatoxin concentration declined rapidly. Degradation of aflatoxin depended primarily on mycelial lysis and high-aeration conditions. Cultures previously reported not to degrade aflatoxin could be induced to do so under these conditions. The percentage and rate of toxin degradation were independent of toxin concentration, and appeared to be nonenzymatic and nonspecific. Degradation simulating that occurring in the fermentor was achieved by reacting aflatoxin with peroxidized methyl esters of vegetable oil; initial degradation was rapid and appeared to involve a complex series of reactions.