Soybean aphid (SA) (Aphis glycines Matsumura) has been an important pest of soybean [Glycine max (L.) Merr.] in the United States since 2000. Identification and genetic characterization of SA resistance in early maturing soybean germplasm will facilitate development of aphid-resistant cultivars in the northern region. To identify new sources of SA resistance in early maturing soybeans, a total of 334 soybean genotypes including resistant and susceptible checks were tested in the greenhouse and field. Caged (no-choice) and noncaged tests were used in greenhouse screening under artificial inoculation of SA, and field evaluations were performed relying on natural aphid infestation with or without artificial SA inoculation. In the greenhouse, four genotypes (pI 603712, pI 464911, pI 430491, and pI 603432B) of maturity group (MG) 0 or 00 exhibited low levels of aphid colonization similar to resistant checks, with 17 to 52 aphids per plant 2 wk after inoculation, and three genotypes (pI 612759B, pI 200595, and pI 603426D) of MG 0 were moderately resistant. In the field, however, only pI 603712 and pI 430491 exhibited a resistance reaction with fewer than 100 or 100 to 200 aphids per plant at peak infestation. plant Introduction 603712 was the only genotype that consistently exhibited resistance to SA in all tests-even higher than that of other known sources of SA resistance in the field. This suggests that pI 603712 might be a new source of SA resistance. In addition, the relatively high levels of SA colonization on a Rag1 genotype (pI 548663 or 'Dowling') in greenhouse tests suggest that the colony used in greenhouse tests might be virulent on Rag1 and therefore might be biotype 2. High levels of SA infestation on Rag1 and Rag2 genotypes in field tests also imply that biotypes 2 and 3 may have been present in the eastern South Dakota field.