Soybean aphid is an economic pest of soybean in North America. Currently, management of soybean aphid is achieved through the use of foliar- and seed-applied insecticides. However, natural enemies play an important role in regulating soybean aphid populations, and may be adversely affected by insecticides. The effects of imidacloprid and thiamethoxam seed treatments on the soybean aphid parasitoid, Aphelinus certus Yasnosh, were examined using a tritrophic bioassay. A. certus was able to parasitize soybean aphids feeding on imidacloprid- and thiamethoxam-treated plants 5 and 6 wk after planting, respectively. However, up to 10 wk after planting, overall parasitism rates were reduced by 69-88% compared with the control. Therefore, neonicotinoid seed treatments may reduce the effectiveness of A. certus as a natural enemy of soybean aphid in seed-treated crops.