The soybean aphid, Aphis glycines was first observed in Wisconsin during the 2000 growing season. Since its introduction many commercial soybean fields have exhibited high levels of aphid activity and yield losses have been widely reported from aphid feeding damage. Field observations of heavy soybean aphid infestations have suggested a potential link between potassium deficiency in soybeans and high aphid numbers. Areas showing symptoms consistent with potassium deficiency were also associated with high levels of soybean aphid infestation. The objective of this study was to examine the effect of potassium availability on plant selection by soybean aphids, and to quantify the resulting impact on soybean yields.