Little information is available concerning the long-term stability of transgene expression in trees. Likewise, the evaluation of transgenic trees modified for insect tolerance or for lignin metabolism has been almost exclusively performed on greenhouse-grown material (Pilate et al., 1997): the effects of transgene expression certainly deserves to be carefully evaluated on real trees planted in the field. Information from field trial studies on these different aspects might be very valuable for evaluation of any potential demand for the plantation of transgenic trees for commercial utilisation that may arise in the future. In order to gain basic information on these questions, applications have been sought in front of the French Commission du Génie Biomoléculaire » for field trial set up with transgenic poplar trees. Two hybrid poplar clones (INRA clone 717-1-B4, Populus tremula x P. alba and INRA clone 353-38, P. tremula x P. tremuloides) transformed with several different constructs have been planted in the field. During these field studies, the containement of any dissemination via vegetative (suckers) or reproductive (flowers) has been stricctly enforced. The first field trial was set up in 1991 with trees transformed with the GUS reporter gene under the control of a constitutive promoter (35S CaMV). After 7 years, these transgenic trees were still expressing the transgene in all their parts (root, stems and buds), the central part of the stem (wet- wood) was the only place where no transgene expression can be detected. Several transgenic flowers were collected and used for controlled pollinations. Transgene appears to segregate in tree progenies following Mendelian law. In 1998, a procedure optimised on regular trees to kill all living parts and prevent any outgrowth have been successfully applied to destroy the transgenic trees. Transgenic aspens over-expressing a cystein proteinase inhibitor and shown to be more tolerant to Chrysomela tremulae (Leplé et al., 1995) were also grown in the field for several years to evaluate their potential effect on target and non-target insect species as well as on their predators and parasites. But unlike what have been previously observed, transgenic trees grown in the field conditions does not seem to be more tolerant towards defoliating insects. Transgenic aspens modified for lignin metabolism (Van Doorselaere et al., 1995: Baucher et al., 1996) by the down- regulation of different key-enzyme for monolignol synthesis are also currently evaluated in field conditions.