Information that describes soybean aphid (Aphis glycines Matsumura) feeding injury effects on soybean [Glycine max (L.) Merr.] yield and seed composition is needed to develop better management practices for this invasive pest. This 2-yr controlled-infestation field study measured aphid populations and the effects of those populations on soybean seed yield, yield components (shoot biomass, pods plant−1, seeds pod−1, and 100 seed weight), and seed composition (oil and protein concentrations) when infested at the vegetative (V5) or reproductive (R2) development stages. In 2003, initial infestation rates of 10, 50, or 100 aphids plant−1 applied at V5 resulted in population peaks of 21,000, 18,000, and 12,000 aphids plant−1 and maximum cumulative aphid-days of 381,000, 327,000, and 242,000, respectively. In 2004, initial infestation rates of 1, 3, 10, 50, or 100 aphids plant−1 applied at V5 resulted in population peaks of 4,600, 9,400, 14,000, 22,000, and 21,000 aphids plant−1 and maximum cumulative aphid-days of 101,000, 229,000, 355,000, 514,000, and 537,000. In both years, the same infestation rates applied at R2 resulted in population peaks and cumulative aphid-day values that were about 42 to 88% lower than the V5 infestation dates. Seed yield, yield components, and seed oil concentration declined linearly as peak aphid numbers plant−1 and maximum cumulative aphid-days plant−1 increased. In contrast, seed protein concentration increased linearly with increasing peak aphid numbers plant−1 Relating these aphid population parameters at the plant growth stages studied enables producers to make informed decisions about the need for and timing of pest management treatments.