Entomologists and plant pathologists have observed for many years that a small green leafhopper scientifically known as Empoasca fabae (Harris) is capable of causing serious injury to a great number of plants, many of which are of economic importance. It is the only known species among leaf hoppers in the United States which causes such widespread destruction and characteristic injury to plants. Scientists working with the insect have speculated on reasons for such pathological results following its attack, and theorized on the nature of its injury. One group has recently proposed that the feeding leafhopper mechanically plugs the vascular elements in the plant and thereby causes the symptoms of injury. The other group adheres to the theory of plant toxaemia resulting from inoculation of a toxin by the insect during its feeding period. There is a need, therefore, to discover fundamental facts which explain the nature of the insect's injury and to determine if the answer lies with one theory or the other, or perchance is a combination of the two.