Certain metals added as salts to a defined basal culture medium influenced the level of aflatoxin production by Aspergillus parasiticus in the low micrograms-per-milliliter range of the added metal. In many cases no change or a relatively small change in mat weight and final pH of the medium accompanied this effect. With zinc at added levels of 0 to 10 mug/ml in the medium, aflatoxin increased 30-to 1,000-fold with increasing of zinc, whereas mat weight increased less than threefold. At 25 mug of added zinc per ml, aflatoxin decreased, but mat weight did not. At an added level of 25 mug or less of the metal per ml, salts of iron, manganese, cooper, cadmium, trivalent chromium, silver, and mercury partly or completelyinhibited aflatoxin production, without influencing mat weight.