The effects of nisin and propionic acid (PA) on aflatoxin production and on mycelial growth and spore germination of the mycotoxigenic fungi Aspergillus parasiticus, A. ochraceus, and Fusarium moniliforme were investigated. The growth of A. ochraceus was completely inhibited on media containing PA with nisin in concentrations of 0.05% PA with 1,000 ppm nisin, and 0.1% PA with 500 or 1,000 ppm nisin. The growth of both F. moniliforme and A. parasiticus was completely inhibited by PA with nisin at a concentration of 0.1% PA with 1,000 ppm nisin. Nisin alone caused a significant increase in mycelial growth when applied to A. ochraceus at 500 or 1,000 ppm and when applied to A. parasiticus at 1,000 ppm. Spore germination of A. ochraceus was completely inhibited on media containing 0.1% PA with 500 or 1,000 ppm nisin. Spores of F. moniliforme failed to germinate in 0.05% PA with 500 or 1,000 ppm nisin, whereas spores of A. parasiticus did not germinate on media containing 0.1% PA with 1,000 ppm nisin. For all three fungi tested, the inhibitory effect on mycelial growth was found to be fungistatic rather than fungicidal. The combined treatment of PA with nisin produced better fungistatic activity than treatment involving either material alone. Nisin, applied alone, did not stimulate aflatoxin production (expressed by μg toxin/mg mycelium), but the combined treatment at certain concentrations was inhibitory to aflatoxin B1 or G1. The production of aflatoxin G1, but not of B1, was stimulated in 0.05% PA with 1,000 ppm nisin and on media containing 0.1% PA with 100 ppm nisin. Nisin is currently applied in foods to prevent spoilage induced by bacteria but not by mold. The results of the present study indicate that a combined treatment of nisin in small concentrations of PA might be useful in preventing mold damage in certain foods and stored grain.