Greenhouse and laboratory experiments were conducted to determine the effect of soil organic matter on the synergistic interaction between nicosulfuron, a sulfonylurea herbicide, and the insecticide terbufos in corn. Terbufos was applied infurrow at planting to soils containing 1, 3, and 5% organic matter and nicosulfuron was applied POST to corn at the three-leaf growth stage. The combination of terbufos and nicosulfuron reduced corn fresh weight 75, 45, and 41 % in soils containing 1, 3, and 5% organic matter, respectively. Terbufos increased [14C]nicosulfuron uptake into corn grown in both 1 and 5% organic matter soils. Terbufos decreased epicuticular leaf wax deposition on corn leaves by 35 and 18% in 1 and 5% organic matter soils, respectively. [14C]Nicosulfuron was completely metabolized after 24 h in the absence of terbufos. In the presence of terbufos, 19 and 11% of intact [14C]nicosulfuron remained 24 h after treatment while 11 and 6% remained after 48 h in the 1 and 5% organic matter soils, respectively. In vivo acetolactate synthase activity was reduced to 3 and 20% of control by nicosulfuron at 24 h and to 6 and 38% of control at 48 h, respectively, in the 1 and 5% organic matter soils that contained terbufos. These experiments indicated that soil organic matter is an important factor in determining the level of injury expected from the synergistic interaction between nicosulfuron and terbufos in corn.