Pest insect aminopeptidase N (APN) is a receptor for Bacillus thuringiensis Cry1 proteins. A 108-kDa APN has been characterised in Spodoptera litura, here their APN three-dimensional structure was determined; it has 4 structural domains. Domain I is the region that recognizes Cry1 toxins, a loop of this section might be very important in this role. Probably, domain II has functions in Cry1 protein-APN interaction. Domain III has a sandwich topology and domain IV is a superhelix. The APN have conserved structures, with slightly mutations just like result of coevolution with Cry proteins.