Resistance to the soybean aphid (Aphis glycines Matsumura) was characterized in segregating populations from crosses of soybean [Glycine max (L.) Merr.] accession PI 71506 to susceptible cultivars, and compared to Rag1 resistance from the cultivar ‘Dowling.’ Two susceptible adapted cultivars were crossed with PI 71506 or Dowling. In no-choice greenhouse assays, resistance corresponded to a single dominant gene model for both the PI 71506-derived and Dowling-derived populations. Segregation of aphid resistance in SD1111RR × PI 71506 F2:3 populations in aphid field-cage trials also fit a single-gene model, as did segregation of aphid resistance in the F2:5 generation. However, other genetic effects may also contribute to aphid resistance from PI 71506. Comparison with Rag1 resistance from Dowling indicated that PI 71506 resistance was weaker than that associated with Rag1, but antixenosis resistance from PI 71506 was effective against an Ohio aphid biotype that has overcome Rag1 resistance.