In France, the chrysomelid beetle Chrysomela tremulae is the main defoliating pest in both young plantations and short rotation stands of poplars. Larvae and adults of C. tremulae feed on leaves of young poplars, leaving only the vein, and can be a limiting factor for biomass production. They also attack the bark when foliage is no longer available, frequently killing the seedlings. In view of the problems concerning the use of insecticides (i.e., prohibitive cost and environmental safety), the introduction into plants of genes confering resistance to this insect is a new promising strategy. The Bacillus thuringiensis-endotoxine gene Cry III A was introduced via cocultivation with Agrobacterium tumefasciens in a Leuce hybrid clone (Populus tremula x P. tremuloides) that is very sensitive to C. tremulae. Three transformants were tested for their effects on survival and development of C. tremulae. C. tremulae survival was significantly reduced on transgenic foliage tested compared to non-transformed foliage. Larval development time was also affected by foliage: it was longer on leaves of the three transgenic than on the non-transgenic control leaves. Male and female pupal wet-weights, measured as an estimate of reproductive potential, were lighter on leaves of the three transgenic foliages. The three transformants assayed on C. tremulae survival and development provided protection to poplar, but this protection was not absolute. The results on mortality suggested that the level of expression of the endotoxin was moderate, but sufficient to induce effects on insect development that will enhance the teratological action.