To better understand disease resistance genes in white poplars, a total of 74 resistance gene homologs (RGHs) with a nucleotide-binding site (NBS) were investigated from the Populus trichocarpa genome, and the expression patterns of these genes in various tissues of a triploid white poplar [(P. tomentosa × P. bolleana) × P. tomentosa] under different growth conditions were examined using quantitative real-time PCR. The results showed that 27 genes were expressed in six of the examined tissues of triploid poplar at various levels. Six, two and three genes were significantly more expressed in apical leaves, young bark and mature bark respectively, than in other tissues, suggesting a potential involvement in tissue specific disease resistance or other functions. Twenty-four genes had dramatically higher expression in apical leaves than in mature leaves, and 22 genes had a higher expression in mature bark than in young bark, suggesting developmental stage-specificity of gene expression in poplar tissues. In addition, wounding induced expression of 22 genes and reduced expression of three genes. Growth in dark conditions caused the up-regulation of 20 genes and the down-regulation of one gene. While methyl jasmonic acid (MeJA) and salicylic acid (SA) induced transcriptional changes of 19 genes, but only one gene was regulated in a simultaneous way. Treatment with the compatible Agrobacterium tumefaciens induced 11 genes. These data suggested that there are complex interconnecting signal transduction pathways regulating the expression of poplar NBS-encoding genes.