Abstract does not appear. First page follows. Introduction In 1938, during an investigation of leafhopper species to determine which were vectors of aster-yellows virus, Xerophloea vanduzeei Law., was tested. This insect was at first thought to be a vector, for after feeding on infected asters it produced, in healthy plants, some symptoms closely resembling those of aster yellows. Further preliminary tests, however, showed the same results even when the insect had been reared entirely on healthy asters and was presumably noninfective for yellows. When the insect was tested on sugar beets, it caused some symptoms resembling those of curly top, even though it had not previously been allowed to feed on curly-top-infected plants. The disease produced by the feeding of this insect is not likely to be of commercial importance on either beets or asters. In twenty-eight years of work on curly top, the senior author has never taken Xerophloea vanduzeei on beets in the field, nor observed it on asters under natural conditions. It has been collected, however, in alfalfa fields. Nevertheless, the fact that diseases so closely simulating curly top and aster yellows could be produced when apparently these viruses are not involved suggests that definite identification of these two virus diseases is not possible from symptoms alone. Furthermore, similar situations may exist with other sucking insects or other syndromes. A brief review of the literature follows. Review of Literature Ball (1918)4 discovered one of the most striking examples of a plant disease, namely hopperburn of potato induced by the potato leafhopper, Empoasca fabae (Harris). He (Ball, 1919) concluded that the explanation of the disease was to be found in “some specific transmitted by the insect,” and that the production of the disease was specific to the potato leafhopper. Eyer (1922a), (1922b) and Fenton and Hartzell (1923) attribute the causative agent of hopperburn to some specific substance in the insect’s body. Ball (1919) attributes hopperburn to an “infection or an injection.” Whether it will prove to be a specific disease like curly top is yet to be worked out. According to Granovsky (19:’28) the symptoms indicate the presence of some infective principle or virulent toxin. Johnson (1934) states that the pathological symptoms caused by the potato leafhopper on forage legumes were not due to the transmission of a virus by this insect.