The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is a serious pest of soybean in the North Central region of the United States. Management tools include resistant varieties and insecticidal seed treatments, used alone or in combination, which may have variable effects on different pest or natural enemy species. In this 1-yr field study we examined the response of soybean aphids and natural enemy abundance to aphid-resistant Rag1 soybeans, with and without thiamethoxam seed treatment. Rag1 resistance, thiamethoxam seed treatment, and both together, significantly reduced cumulative aphid days and increased yield compared to the untreated susceptible control, though there was no interaction of these factors. Peak aphid density in the control was 799.4 ± 174.8 aphids/plant. In Rag1 resistant soybeans, peak aphid density (312.2 ± 121.3 aphids/plant) was above the economic threshold but below the economic injury level, whereas in susceptible soybeans with seed treatment, the peak aphid population (659 ± 164.9 aphids/plant) exceeded the economic threshold and approached the economic injury level. Yield and cumulative aphid days in resistant soybeans and seed-treated soybeans did not differ. The fewest cumulative aphid-days and highest yield were obtained from seed-treated resistant soybeans, where peak aphid density was 39.2 ± 9.2 aphids/plant. Natural enemy abundance was positively correlated with aphid abundance. Rag1 resistant soybeans and thiamethoxam-treated soybeans were found to have lower natural enemy populations than were present in the control. Harmonia axyridis Pallas and Orius insidiosus Say were the dominant natural enemies in the study. These results are from one season of study in this system and should be interpreted in that light.