Corn plant roots exposed to amine salt of 2,4-dichlorophenoxyacetic acid at dosages of 5, 20, 80, and 320 ppm applied to the soil surface in pots were infested with the corn leaf aphid,Rhopalosiphum maidis(Fitch), and injected with the common com smut pathogen,Ustilago maydis(DC.) Cda, in the laboratory. Aphids were about 45% more abundant on com plants treated with 5 and 20 ppm (20 ppm equivalent to about 1 liter of 2,4-D formulation/ha) than on the untreated corn plants. The weight of common corn smut galls on corn plants treated with 20 and 80 ppm was more than twice that of galls on untreated plants.