Extensive planting of Bacillus thuringiensis(Bt)-transgenic plants economically benefits society; however, the potential risk they pose is receiving increasing attention. This study used enzyme-linked immunosorbent assay and fluorescence quantitative PCR(RT-PCR) to monitor the temporal and spatial dynamics of the expression of Bt toxic protein in a forest of 6- to 8-year-old trees of transgenic insect-resistant poplar 741 for three consecutive years. The enrichment, distribution, and degradation of Bt toxic protein and the influence of transgenic poplars on the targeted insect population, Hyphantria cunea, were investigated. The content of Cry1Ac toxic protein dynamically changed in transgenic poplar. During the annual growth cycle, the content initially increased, then decreased in the long and the short branches of the crown and in the root system, peaking in August. During the study, the protein did not accumulate overtime. The m RNA transcription of gene Cry1Ac was almost consistent with the level of the protein, but transcription peaked in July. In the transgenic and control forestland, microscale levels of the Cry1Ac toxic protein were detected from the soil, but increased accumulation was not observed with the planting year of transgenic poplar. Meanwhile, Bt was isolated and detected molecularly from the soil in the experimental forestland. A systematic investigation of the density of H.cunea in the experimental transgenic poplar forest indicated that transgenic Pb29 poplar could resist insects to a certain degree. At peak occurrence of the targeted insects,the density of H. cunea in the experimental forest was significantly lower than in the nontransgenic poplar forest. Extensive planting of Bacillus thuringiensis (Bt) -transgenic plants economically benefits society; however, the potential risk they pose is receiving increased attention. This study used enzyme-linked immunosorbent assay and fluorescence quantitative PCR (RT-PCR) to monitor the temporal and spatial dynamics of the expression of Bt toxic protein in a forest of 6- to 8-year-old trees of transgenic insect-resistant poplar 741 for three consecutive years. The enrichment, distribution, and degradation of Bt toxic protein and the influence of transgenic poplars on the targeted insect population, Hyphantria cunea, were investigated. The content of Cry1Ac toxic protein dynamically changed in transgenic poplar. During the annual growth cycle, the content initially increased, then decreased in the long and the short branches of the crown and in the root system, peaking in August. During the study, the protein did not accumulate overtime. The m RNA transcription of gene CrylA was almost consisten t with the level of the protein, but transcription peaked in July. In the transgenic and control forestland, microscale levels of the CrylAc toxic protein were detected from the soil, but increased accumulation was not observed with the planting year of transgenic poplar. Meanwhile, Bt was isolated and detected molecularly from the soil in the experimental forestland. A systematic investigation of the density of H. cunea in the experimental transgenic poplar forest indicated that transgenic Pb29 poplar could resist insects to a certain degree. At peak occurrence of the targeted insects , the density of H. cunea in the experimental forest was significantly lower than in the nontransgenic poplar forest.