Several lines of evidence have indicated that lipoxygenase enzymes (LOX) and their products, especially 9S- and 13S-hydroperoxy fatty acids, could play a role in the Aspergillus/seed interaction. Both hydroperoxides exhibit sporogenic effects on Aspergillus spp. (Calvo, A., Hinze, L., Gardner, H.W. and Keller, N.P. 1999. Appl. Environ. Microbiol. 65: 3668-3673) and differentially modulate aflatoxin pathway gene transcription (Burow, G.B., Nesbitt, T.C., Dunlap, J. and Keller, N.P. 1997. Mol. Plant-Microbe Interact. 10: 380-387). To examine the role of seed LOXs at the molecular level, a peanut (Arachis hypogaea L.) seed gene, PnLOX1, was cloned and characterized. Analysis of nucleotide sequence suggests that PnLOX1 encodes a predicted 98 kDa protein highly similar in sequence and biochemical properties to soybean LOX2. The full-length PnLOX1 cDNA was subcloned into an expression vector to determine the type(s) of hydroperoxide products the enzyme produces. Analysis of the oxidation products of PnLOX1 revealed that it produced a mixture of 30% 9S-HPODE (9S-hydroperoxy-10E, 12Z-octadecadienoic acid) and 70% 13S-HPODE (13S-hydroperoxy-9Z, 11E-octadecadienoic acid) at pH 7. PnLOX1 is an organ-specific gene which is constitutively expressed in immature cotyledons but is highly induced by methyl jasmonate, wounding and Aspergillus infections in mature cotyledons. Examination of HPODE production in infected cotyledons suggests PnLOX1 expression may lead to an increase in 9S-HPODE in the seed.