Technical Abstract: The greenbug is a serious aphid pest of wheat and sorghum in the southern High Plains of the US. The greenbug resistant gene Gb3 originated from the goatgrass has shown consistent and durable resistance against prevailing greenbug biotypes in wheat fields for moer than 30 years. Our goal is to clone this aphid resistance gene through the map-based cloning strategy. We previously placed the Gb3 gene in a recombination-rich region of wheat chromosome arm 7DL. The very large genome of bread wheat causes a significant challenge to clone this important gene. So we took a different strategy to achieve this goal by working in the diploid species foatgrass. In this study, high–resolution genetic mapping was carried out using an F2:3 segregating population derived from two goatgrass accessions, the resistant PI 183967 (original donor of Gb3) and susceptible AL8/78..Molecular markers were developed by exploring all available resources including wheat RFLPs, SSRs, ESTs and the wheat D genome physical maps (BAC contigs). Forty five PCR-based markers were developed and mapped to the chromosomal region spanning the Gb3 locus. Two closest flanking markers defined a region of 1.1 cM genetic length surrounding the resistance gene. This high-resolution genetic map with markers closely linked to Gb3 layspresents a solid foundation for isolation of this aphid resistance gene and marker-assisted selection of this gene in wheat breeding.