Preplanting treatment of filter paper or soil containing α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin) with any of a wide variety of plant lipids prevented the phytotoxic action of the herbicide in laboratory situations. The protective action of externally applied lipids was attributed to preventing sorption of trifluralin by seedlings. However, protection by internal lipids also appeared likely. A significant correlation (1% level) was demonstrated between seedling sensitivity to trifluralin and the lipid content of dry seeds of 11 weed and crop species. Artificially induced increases in the internal lipid content of wheat (Triticum aestivum L. ‘Mediterranean’) seed resulted in greater tolerance of seedlings to trifluralin. Therefore, it is hypothesized that selective phytotoxicity of trifluralin to young seedlings is determined in part by the amount of endogenous lipids available to trap trifluralin and keep it from its site of phytotoxic action. Tolerance above that predicted from seed lipid content was observed in a twelfth species of the study.