Granular formulations of phorate at 3 lb active ingredient, and Di-Syston® (O, O-diethyl S-[2- (ethylthio) ethyl] phosphorodithioate) at 2 lb and 4 lb active ingredient, increased potato yield significantly when applied as plants were breaking through the soil. Phorate at 1.5 lb gave an increased yield, but was not significantly better than the check. Sprays of DDT at lib, Bayer 25141 (O, O-diethyl O-p- (methylsulfinyl) phenyl phosphorothioate) at 1 lb, and Thiocron® (O,-dimethyl-S- (2-methoxyethyl-carbamoylmethyl) dithiophosphate) at 0.25 lb gave significantly greater yields than the check. All sprayed plots were treated 4 times at 7-day intervals and produced somewhat lower yields than the systemic insecticide-treated plots except phorate at 1.5 lb per acre. Populations of adult potato leafhoppers, Empoasca !Soarfabae (Harris), were lower in sprayed plots, but evidence of hopper burn was less evident in plots treated with systemic insecticides. Chemical analyses of potato tubers showed less than 0.02 ppm in phorate-treated plots (3 lb) and 0.08 to 0.09 ppm in Di-Syston-treated plots (4 lb). In Canada the tolerances are zero for phorate and 0.2 [or Di-Syston.