The genus Populus exhibits very interesting characteristics which make poplar species highly suitable for potential use in short rotation intensive cultures. Unfortunately, most of the clones available exhibit a high sensitivity to insect attack. Recent advances in gene transfer and recombinant DNA methods provide now the opportunity for improving insect resistance in poplars. We report here recent results on the expression of a cry3A delta-endotoxin synthetic gene in transgenic poplars and its high toxicity against Chrysomela tremulae, the main coleopteran pest for young poplar plantations in France. The INRA clone 353-38 (Populus tremula x Populus tremuloides) was chosen as a model tree due to its ability to Agrobacterium mediated transformation and its high sensitivity to insect attack. The presence of AT-rich nucleotide sequences in native cry genes are responsible for high instability or low level of expression of the transgene in dicodyledonous plants, so poplar plants were transformed with synthetic cry3A gene under the control of the mannopine synthase promoter (kindly provided by the Calgene Company). Based on northern and western blot analyses, two transgenic lines among 25 were selected for the highest expression of cry3A gene. A developmental test on newly emerged Chrysomela tremulae larvae was performed on those selected lines. A high level of mortality was monitored in laboratory conditions (more than 90%). These results open the way to develop experiments at lab and field scale to evaluate how durable can be such a specific resistance in a tree species.