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The greenbug, Schizaphis graminum ( Rondani) is an extremely damaging aphid pest of barley ( Hordeum vulgare L.) particularly in the southern Great Plains of the USA. The simply inherited, dominant resistance gene Rsg1 is in all greenbug-resistant US barley cultivars. In this study, we conducted molecular mapping of Rsg1 using an F2:3 population derived from a cross between the greenbug-resistant Post 90*4/ R015 and susceptible CI2260 inbred lines. Segregation of host responses to greenbug biotype E infestation confirmed that a single dominant gene is responsible for greenbug resistance in Post 90*4/ R015. Simple sequence repeat ( SSR) markers evenly distributed along the seven barley chromosomes were employed for the construction of a framework genetic map. Linkage analysis placed the Rsg1 locus in the long arm of chromosome 3 H (3 HL) flanked by SSR markers Bmag0877 and GBM1420 that were 35 c M apart. Polymorphic single-nucleotide polymorphism ( SNP) markers in 3 HL were identified from an Illumina Golden Gate SNP assay and used for targeted mapping to locate Rsg1 to an 8.4-cM interval. Comparative analysis identified syntenic genomic regions in Brachypodium distachyon chromosome 2, in which 37 putative genes were annotated including a NB- LRR-type resistance gene homologue that may be a potential candidate gene for the Rsg1 locus of barley. Results from this study offer a starting point for fine mapping and cloning of this aphid resistance gene in barley. [ABSTRACT FROM AUTHOR]