The soybean aphid, Aphis glycines Matsumura, was recently (2000) introduced from Asia and has become a serious soybean pest in North America. Many approaches have been attempted to control this pest, however so far chemical control has been the most reliable. Although neonicotinoid insecticide seed treatments, imidacloprid and thiamethoxam, have been suggested as a method of control, the efficacy and duration of such treatments is in question. Soybean aphid populations were monitored during two field seasons and yield effect determined among imidacloprid and thiamethoxam treated seeds when compared to untreated plots. The field study was complemented by laboratory and greenhouse studies. A reliable bioassay technique was developed to determine systemic insecticidal activity including both lethal and sublethal effects. Imidacloprid and thiamethoxam were toxic and reduced population growth at very low concentrations (LC₅₀s 31.29 and 16.91 and EC₅₀s are 2.29 and 3.54 ηg/ml). Sublethal effects were observed as low as 10 and 10.3 ng/ml, imidacloprid and thiamethoxam, respectively. A greenhouse experiment was also conducted in which aphids were transferred to trifoliates in different soybean vegetative stages and mortality was determined. These and further results from field and laboratory studies will provide information regarding the efficacy of seed treatments with imidacloprid and thiamethoxam against soybean aphid.