Lignin, a major component of the plant cell wall, is an undesirable compound for the pulp and paper industry and its removal during kraft pulping is costly and polluting. In order to obtain trees with improved pulping performances, genetic engineering experiments have been targetted in poplar to the gene coding for the cinnamyl alcohol dehydrogenase (CAD), the last enzyme of the monolignol biosynthetic pathway. Previously, transgenic aspen (Populus tremula x P. alba) underexpressing CAD enzyme (20% residual activity) have been produced and characterized for their specific lignin profiles and pulping behaviour (1-3). In this work, we will illustrate the optimization of genetic transformation on hybrid poplar (P. deltoides x P. nigra), cv. 'OGY' and the subsequent production of CAD down-regulated 'OGY plants. This transgenic material exhibit extremely low CAD activity (2 to 3% residual activity) and striking alteration in lignin content and composition. 10 to 15% reduction in Klason lignin have been repeatedly recorded on young greenhouse-grown transgenic 'OGY' plants. Interestingly, the S/G ratio is significantly decreased, and new compounds are consistantly detected in the lignin polymer of the CAD down-regulated transgenic lines. Moreover, mild alkaline hydrolysis of extractive free wood performed on this material allow to remove a high amount of lignin, more than twice as much as in the control untransformed plants. This last observation appears very promising for application in the delignification process during Kraft pulping. This work was carried out in the framework of the EU research program FAIR-TIMBER (CT95-0424).