The ether extract of small everlasting (Antennaria microphylla Rydb.) was phytotoxic to lettuce seed root growth and leafy spurge (Euphorbia esula L. # EPHES) root elongation and cell culture growth. Three phenolic compounds (hydroquinone, arbutin, and caffeic acid) were chromatographically isolated and characterized from the ether extract. Arbutin and caffeic acid were moderately phytotoxic to leafy spurge root growth at 300 ppm (w/v), and hydroquinone was strongly phytotoxic to leafy spurge root growth at 50 ppm (w/v). The observed phytotoxicity of hydroquinone and the high-yield natural occurrence of arbutin, a water soluble, easily hydrolyzed monoglucoside of hydroquinone, in small everlasting is consistent with the participation of these two compounds in the observed allelopathy of small everlasting against leafy spurge.