Chrysomela tremulae is one of the most threatening leaf-feeding beetles in young poplar plantations and nurseries in France. Adults and larvae feed on young poplar leaves and destroy the new growth. They cause reductions in biomass or the death of young nursery plants and decrease the biomass production of short rotation coppices. Previous studies have revealed significant differences in responses among poplar species and hybrids, and knowledge on the host plant resistance and its genetic variation is a first step to achieve selection for durable resistance. To determine the genetic basis of resistance to C. tremulae, an interspecific Fl family P. deltoides x P trichocarpa of 330 individuals was used. The P. deltoides parent was resistant and the male parent P. trichocarpa suceptible. The evaluation of resistance was carried both in laboratory and field conditions on respectively 330 and 184 genotypes. The preferences of C. tremulae were determined (1) in the laboratory by evaluation of adults feeding through multiple choice tests on leaf disk (2) in the field, by visual notations of damage in a randomised plantation. It allowed to classify the progenies according to their level of sensivity. The distributions of clonal means observed fit with a polygenic control for the traits. Resistance to C. tremulae showed a higher broad sense heritability in the laboratory tests than in the field evaluation. The genetic maps of the 2 parents : the female parent P. deltoides and the male parent P. trichocarpa realized, using RAPD, RFLP, AFLP and SSR markers were used to understand the genomic structure of resistance. Based on 'interval mapping' we have identified for laboratory test 3 QTL, explaining between 6.7% and 14.5% of clonal differences on P. deltoides and P. trichocarpa linkage groups.