Root exudates of Sorghum bicolor consist primarily of a dihydroquinone that is quickly oxidized to a p-benzoquinone named sorgoleone. The aim of this investigation was to determine the potential activity of sorgoleone as an inhibitor of weed growth. Bioassays showed 125 tiM sorgoleone reduced radicle elongation of Eragrostis tef. In liquid culture, 50-#M sorgoleone treatments stunted the growth of Lemna minor. Over a 10-day treatment period, 10 tzM sorgoleone in the nutrient medium reduced the growth of all weed seedlings tested: Abutilon theophrasti, Datura stramonium, Amaranthus retroflexus, Setaria viridis, Digitaria sanguinalis, and Echinochloa crusgalli. These data show sorgoleone has biological activity at extremely low concentrations, suggesting a strong contribution to Sorghum allelopathy.