Foliar rust diseases causes by Melampsora spp. have the potential to be very damaging to poplar plantations. The most effective method of reducing the risk is to select varieties that are rust tolerant. Plants respond to microbial attack by activating an array of inducible defense responses. Poplar molecular responses to Melampsora spp. infection are however poorly understood. To characterize the response of Populus trichocarpa x deltoides cv. Beaupré to virulent (98AG31) and avirulent (93ID6) strains of M. larici-populina and identify genes that may play a role in resistance, expression profiling of plants have been carried out. Approximately 200 (4.3%) of the ~ 4600 genes monitored showed reproducible and significant (t-test P>0.05) expression level changes in at least one of the interactions. In the incompatible interaction, defense- and stress12th New Phytologist Symposium 19 response genes (e.g., PR-2, PR-3, PR-5, PR-10) showed an increase in transcripts which peaked 48h post-contact. Decreased levels of transcripts encoding primary metabolism, photosynthesis and photorespiration enzymes was observed in both interactions. Expression profiling technologies, in combination with other tools, such as RNAi, will have a substantial impact on our understanding of poplar-rust interactions and defense signaling pathways.