Technical Abstract: There is little published information available that describes the effects of soybean aphids (Aphis glycines Matsumura) on soybean [Glycine max (L.) merr] growth, yield, and seed composition. The objective of this research was to measure how soybean growth, yield, and yield components are affected by soybean aphid infestations under controlled environments. Greenhouse-grown plants were infested with aphids at the first node stage (V1), the third node stage (V3), the beginning bloom stage (R1), the full pod stage (R4) and the beginning maturity stage (R7). Aphids remained on the plants for a total of 6000 aphid-d (e.g. 600 aphids present for 10 days) after which the insects were removed with insecticide. Increases in stem length, as compared to uninfested controls, were observed in plants infested at the V1 and V3 stages but not at the R7 development stage. Aphid infestation had no significant effect on seed yield, number of pods plant-1, or number of seeds pod-1. Aphid treatment reduced seed number plant-1. Individual seed weights were greater in aphid infested plants than control when treated at the V1 and V3 development stage but were not different when aphids were applied to plants in the reproductive stages. Seed protein concentration was not affected by aphid infestation treatment. Infestation at the R4 and R7 stages resulted in dramatic reductions in seed oil concentrations compared with controls. Even though aphid injury in this study did not lead to a significant loss in total seed yield, aphid-induced reductions in oil concentration suggest that seed composition responses to aphid injury should be an important consideration when conducting research to develop IPM systems for this soybean pest.