Provided herein are isolated nucleic acid molecules representing a genetically defined region of the genome of the aphid resistant soybean plant (Glycine max) cultivar Dowling that confers resistance to soybean aphid (Aphis glycines). Within the region is a gene encoding the aphid resistance protein Rag1. Rag1 aphid resistance amino acid sequences are also provided. Also provided herein are methods for conferring aphid resistance on a plant or enhancing aphid resistance in a plant by transforming it to contain and express such nucleic acid sequences encoding Rag1 aphid resistance or introgressing DNA encoding the trait into the plant by plant breeding. Further provided are polymorphic markers useful for identifying plant germplasm containing aphid resistance, and methods for makings such markers.