Three expts. were conducted under 3 distinct sets of conditions which made possible the evaluation of materials for leaf-hopper control alone, for disease control alone, and for joint leafhopper and disease control. The data obtained show the following: DDT gave excellent leafhopper control and high yield responses; DDT has no fungicidal value; DDT is non-injurious to potatoes when used alone or in combination with the fungicides tested; DDT and several inorganic and organic fungicides are compatible since each material was as effective when used in combination as when employed alone. The excellent growth of plants sprayed with DDT under conditions of leafhopper infestation suggests that DDT might be a growth promoter. The better growth, however, may be simply the result of leafhopper control, for when leaf-hoppers are not present plants sprayed with DDT are no different in appearance from those unsprayed or sprayed with fungicide alone. It is apparent, then, that leafhoppers have a more serious effect on plants than has heretofore been thought. Fungicides used were Bordeaux, Compound A, Zerlate, and Dithane plus zinc sulfate-lime. Deenate was used for DDT.