The leafhopper, Macropsis trimaculata, is reported to be an inefficient vector of peach yellows and little peach. Since the viruses causing these diseases are inactivated when infected peach tissues are exposed to temps. as low as 95[degree] F, it is suggested that they may also become inactivated when infective leafhoppers are exposed to high summer temps. and that such inactivation may explain the inefficiency of the vector. M. t. was not available in sufficient numbers during the past 2 yrs. to test this hypothesis, but, as the aster yellows virus is also sensitive to high temps., the aster leafhopper, Macrosteles divisus (Cicadula sexnotata) was substituted for Macropsis trimaculata in heat inactivation expts. The expts. gave support to this hypothesis. The aster leafhopper invariably lost the ability to transmit aster yellows when exposed to 90[degree] F or higher for 1 day or longer. Infective insects exposed to this temp. for as long as 12 days lost and never regained ability to transmit unless subsequently allowed to feed on a diseased aster plant. When exposed for less than 12 days, they always regained ability to transmit after a waiting period which varied with the length of the treatment[long dash]an indication that the aster yellows virus multiplies in the aster leafhopper.