Aflatoxins are highly toxic and carcinogenic compounds produced by certain Aspergillus species on agricultural commodities. The presence of fatty acid hydroperoxides, which can form in plant material either preharvest under stress or postharvest under improper storage conditions, correlates with high levels of aflatoxin production. Effects on fungal growth and aflatoxin production are known for only a few of the numerous plant metabolites of fatty acid hydroperoxides. Jasmonic acid (JA), a plant growth regulator, is a metabolite of 13-hydroperoxylinolenic acid, derived from alpha-linolenic acid. The volatile methyl ester of JA, methyl jasmonate (MeJA), is also a plant growth regulator. In this study we report the effect of MeJA on aflatoxin production and growth of Aspergillus flavus. MeJA at concentrations of 10(-3)-10(-8) M in the growth medium inhibited aflatoxin production, by as much as 96%. Exposure of cultures to MeJA vapour similarly inhibited aflatoxin production. The amount of aflatoxin produced depended on the timing of the exposure. MeJA treatment also delayed spore germination and inhibited the production of a mycelial pigment. These fungal responses resemble plant jasmonate responses.