The soybean aphid (Aphis glycines) has rapidly spread through the Midwest causing consistent yield losses since its introduction. In an effort to find sources of resistance to this pest, several plant introductions have been screened in controlled laboratory conditions or in few locations under field conditions. In order to better characterize different soybean resistant lines we measured resistance on 18 soybean varieties at different locations across the Midwest in two years. Clustering of lines with similar performance was conducted. Correlation analyses between aphid pressure determined by different locations and aphid infestation of groups of lines as determined by the cluster showed no genotypic by environment interaction. Furthermore, we selected one soybean line with known source of resistance (Rag1 gene) and a related line without the Rag1 gene. Consistency of the resistance conferred by Rag1 was tested in terms of yield losses along different levels of aphid infestations at field conditions. Even though the resistant line showed high levels of aphid infestation it did not show yield losses. These results suggest that the Rag1 gene may confer not only resistant to soybean aphid (aphid numbers per plant) but also tolerance to this pest (not significant yield loss when infested). In an effort to understand the Rag1 mechanism of resistance we tested for differences in free amino acid composition of leaves in the Rag1 genotype and its susceptible related line at three soybean developmental stages. We detected constitutive differences in amino acid composition between these two lines, and also differences in induced responses in the presence of aphids. The amino acids detected in this study could be selected to evaluate their putative role in aphids growth and reproduction.