Hybrid poplars (Populus tremula × Populus tremuloides) have been genetically engineered via Agrobacterium tumefaciens, to express a synthetic cry3Aa gene derived from the native Bacillus thuringiensis subsp. tenebrionis cry3Aa gene. The presence and the expression of the transgene have been verified in four transgenic poplar lines, using Southern, northern and western analyses. The transgenic poplar's toxicity towards the phytophagous beetle Chrysomela tremulae (Coleoptera, Chrysomelidae) has been assessed on six month-old greenhouse-grown selected plants in laboratory conditions. Laboratory experiments consisted of feeding tests of fresh detached leaves on C. tremulae at all developmental stages. Our results indicate that the transgenic poplar leaves, expressing a Cry3Aa protein amount in a range of 0.05?0.0025% of total soluble protein, were definitely deleterious for C. tremulae, regardless of the developmental stage.