The barley stem rust resistance gene rpg4 was physically and genetically localized on two overlapping BAC clones covering an estimated 300-kb region of the long arm of barley chromosome 7(5H). Initially, our target was mapped within a 6.0-cM region between the previously described flanking markers MWG740 and ABG391. This region was then saturated by integrating new markers from several existing barley and rice maps and by using BAC libraries of barley cv. Morex and rice cv. Nipponbare. Physical/genetic distances in the vicinity of rpg4 were found to be 1.0 Mb/cM, which is lower than the average for barley (4 Mb/cM) and lower than that determined by translocation breakpoint mapping (1.8 Mb/cM). Synteny at high resolution levels has been established between the region of barley chromosome 7(5H) containing the rpg4 locus and the subtelomeric region of rice chromosome 3 between markers S16474 and E10757. This 1.7-cM segment of the rice genome was covered by two overlapping BAC clones, about 250 kb of total length. In barley the markers S16474 and E10757 genetically delimit rpg4, lying 0.6 cM distal and 0.4 cM proximal to the locus, respectively.