Population growth of the soybean aphid (Aphis glycines Matsumura) was studied under elevated atmospheric carbon dioxide. Soybean aphids were placed on soybean plants grown under 550 ppm CO₂ or current ambient atmosphere (370 ppm) for a period of one week. Aphid populations living on plants grown under elevated CO₂ significantly increased, roughly 16 times, while populations living on control plants only increased by half that amount. Changes in aphid population sizes were hypothesized to be correlated to leaf temperature under the two atmospheric treatments. Plants grown under elevated CO₂ have reduced stomatal conductance and a denser canopy because of increased size. Both of these factors lead to increases in leaf temperature. As soybean aphids may live within the boundary layer of soybean leaf surfaces, their temperatures may also be elevated leading to faster growth and reproduction.