Leaf rust caused by Melampsora larici populina (Mlp) is a major disease for poplar trees in Europe. Using a segregating population populus deltoides x P. trichocarpa, Rus controlling size of the uredinia has been characterized. Rus, inherited from P. trichocarpo, explains between 42.4% and 82.8% of the phenotypic variation to different Mlp strains. We describe the construction of a fine genetic map using a segregating population of 1410 individuals with available SSR and markers derived from an AFLP-BSA approach. The alignment with the public available genome sequence (http://genome.jgi-psf.org/Poptr1 1/Poptr1 1.home.html) allowed locating Rus on a cluster of resistance genes in the linkage group XlX. To narrow down genomic region corresponding to the Rus locus, additional genetic markers including SSR, STS corresponding to positional candidate genes were developed. Seven hundred and thirteen more individuals were used to perform a high resolution mapping of the region. At this term, Rus is localised within a 0.5 cM interval between the closely genetic markers SRR_84 and L29_1. To physically clone the gene, a 14X BAC library of the male P. trichocorpa has been constructed. Chromosome walking allowed the construction of two local physical maps corresponding to the two haplotypes Rus VS rus. Two BAC clones carrying Rus were selected and sequenced. Rus was defined into a 45kb interval.