Recent reviews [Crafts, Currier, Woodford] dealing with the mode of action of phytotoxic chemicals claim the acute toxicity of petroleum oils to plant tissues is due to the destruction of the plasma membrane of the cell. Although considerable evidence supports such a claim, much of it rests on results from the effect of vapors on greenhouse grown plants rather than on the effect of liquid oils on field grown plants. Of the numerous workers who have noted a selective action of oils on different plant species, most of them determined the selectivity by visual observation of injury. Currier and Peoples, Dallyn, and Van Overbeck and Blondeau linked the selective action by experimentation to a differential effect on a cellular level but only the work of Currier and Peoples included direct determinations of cellular vitality. It seemed desirable, therefore, to obtain further information on the relative susceptibility to petroleum oils of field-grown plants.