The soybean aphid, Aphis glycines Matsumara, was discovered in the United States in the summer of 2000. Since that initial discovery, the aphid has spread across northern soybean production regions. In 2001, we examined the physiological responses of soybeans to low aphids densities (fewer than 50 aphids/leaf). In this study, we determined photosynthetic rates, leaf fluorescence responses, and photosynthetic responses to variable carbon dioxide and light levels. In addition, analyses for chlorophyll content and stable carbon isotope ratios were used to differentiate potential differences in stomatal versus mesophyll limitations to photosynthesis. We observed rate reductions of up to 50% on infested leaflets, including lealets with no apparent symptoms of aphid injury (such as chlorosis). Differences in fluorescence data indicated that photoelectron transport was not impaired. These results indicate that substantial physiological impact on soybean is possible even at low aphid densities. Also, the conventional view of aphid injury acting through reductions in chlorophyll content and light-harvesting reactions of photosynthesis is not supported by our findings in this system.