The recent advances in gene transfer and recombinant DNA methods provide opportunities for improving woody plants by introducing foreign genes encoding important traits such as herbicide resistance, pathogene attacks protection or wood quality. In France, two coleopterous, Chrysomela populi and Chrysomela tremulae, which feed on poplar leaves, are responsible for most of the damage on poplar plantations. An analysis of proteolytic activity in the gut of Chrysomela tremulae larvae revealed cysteine proteinases as the major digestive enzymes. The rice cysteine proteinase inhibitor OCI was shown in vitro to inhibit most of this proteolytic activity. In order to assess the potential utilization of OCI as a Chrysomela control agent, the OCI coding sequence fused to a strong constitutive promoter was transferred into a hybrid poplar clone (Populus tremula X P. tremuloides) highly sensitive to Chrysomela attack. Gene transfer methods using Agrobacterium tumefaciens and plant regeneration systems have been developed for this hybrid clone. Transgenic poplars were analyzed both for their ability to express the active form of OCI and for toxicity against Chrysomela tremulae. OCI prótein of the expected electrophoretic mobility, antigenicity and inhibitory activity was detected in transgenic poplars. Three transformants were used as feeding material for in vitro insect bioassays. All of these poplars were shown to have a toxic effect on larvae growth and development compared to non-transformed control plants. A maximum of 89% of global mortality was obtained for one transgenic clone. These results give additional evidence of the promising use of such proteinase inhibitors as potential tools for insect control strategies using genetically engineered plants.