Abstract:In order to exploring the binding of Bt toxin with the midgut of larvae of Helicoverpa armigera Hübner, the fusion gene of cry1Ac and green fluorescent protein (GFP) was constructed by overlap-PCR, and expressed in Escherichia coli. It was found that Cry1Ac-GFP fusion protein expressed in E.coli emit green fluorescence under blue light. Bacteria solution containing Cry1Ac-GFP fusion protein was mixed into artificial diet, on which 3rd instar larvae of H.armigera were raised for 96 hours. After turned to frost slice, midgut was observed under blue light of fluorescence microscope. It was showed that midgut holding Cry1Ac-GFP fusion protein emits strong green fluorescence. Comparing the fluorescence position in midgut of larvae between Bt resistant strain and susceptible strain we found that peritrophic membrane of susceptible strain disappeared, intestinal wall cells was labeled with Cry1Ac-GFP fusion protein and emitted strong fluorescence instead. However, the Cry1Ac-GFP fusion protein only accreted with peritrophic membrane of resistant strain and showed fluorescence, which suggested that the binding of peritrophic membrane with Cry1Ac toxin protein may result in resistance of larvae of H.armigera to Cry1Ac insecti- cidal crystal proteins.