Technical Abstract: Stilbene phytoalexins are produced by the peanut plant as a defense response to exogenous stimuli, particularly when challenged by fungal invasion. The plant's ability to produce phytoalexins under unchallenged conditions has not been reported. Phytoalexins possess antifungal activity against Aspergillus, Penicillium and Cladosporium species, and are suggested to play an important defensive role at early stages of plant development. The purpose of this work was to characterize phytoalexin production by different parts of the developing peanut plant. The concentrations of stilbene phytoalexins varied from 10 ng/g of t-resveratrol in the hypocotyl to 12 µg/g of the major unknown stilbene in the root mucigel. At least 5 new stilbenes were detected in the mucigel; their structural elucidation is in progress. Two major bound phenolic acids, ferulic and p-coumaric, were present in high concentrations in the root, the hypocotyl and the shoot; p-coumaric acid was found in cotyledons. Three different peanut genotypes (Georgia Green, Valencia and Virugard) produced the same set of stilbene phytoalexins and bound phenolic acids. The mucigel contained low-polar stilbenes, but none of the known phytoalexins such as the arachidins and t-resveratrol. The hydrophobic nature of the mucigel stilbenes suggests that they may have an affinity for fungal membranes, the usual site of phytoalexin action. Besides lubricating the root and enhancing soil quality, the peanut root mucigel may serve as an active protective barrier for soil fungi due to the presence of phytoalexins. Although ferulic and p-coumaric acids are typically associated with dormancy, these phenolic compounds also may protect the plant against pathogenic fungi.