The differential expression of genes induced by an acute ozone treatment was analysed in two poplar clones, i.e. Populus deltoides x maximowiczii, Eridano clone, and Populus x euoramericana, I-214 clone, respectively, sensitive and tolerant to this pollutant, performing suppression subtractive hybridization (SSH). From the obtained cDNA libraries several clones were obtained, which corresponded to differentially regulated genes. Preliminary expression analyses of four genes, Fs23A-LRP, Ft33B-CaBP, Ft312B-WRKY, and Ft32C-WAK identified by the primary screening, were conducted by semi-quantitative RT-PCR to evaluate the ozone responsiveness of the libraries. The most interesting finding is the co-activation of a wall associated kinase and a WRKY transcription factor in response to O(3) stress.