Aminopeptidase N (APN) is a specific receptor of Bacillus thuringiensis Cry toxins. Characterization of the APN produced in insect midgut and functional studies are essential to understand the mode of action of insecticidal Cry toxins. My dissertation project focused on a component toxin of B.t. subsp. jegathesan, Cry11Ba, and a mosquito species Anopheles gambiae that is highly susceptible to Cry11Ba. The specific aims of my research were to: 1) characterize the structure, localization and expression of APNs in larval mosquitoes, 2) assess whether APNs specifically interact with Cry11Ba in mosquito midguts, and 3) determine the binding sites of Cry11Ba on APNs. Annotation of genomic and EST sequences of An. gambiae identified four APN genes, AgAPN2, -3, -4 and -5 with high sequence similarities. The cDNA of AgAPN2 was cloned and polyclonal antibodies against AgAPN2 were generated. Mass spectrometry analyses of purified APNs indicated three APNs, AgAPN2, -3 and -5 were expressed in larval mosquito midguts. Immunohistochemistry confirmed the localization of APNs on the brush border membrane (BBM) of epithelial cells in the posterior midgut. Sequence analyses of AgAPN2 revealed a characteristic zinc-binding motif (HEIAH), three potential N-glycosylation sites and a predicted glycosylphosphatidylinositol (GPI) anchor site. Functional studies indicated Cry11Ba affinitively precipitated AgAPN2 among other GPI-anchored proteins released by phosphatidylinositol-specific phospholipase C (PI-PLC) from An. gambiae brush border membrane vesicles (BBMVs). A 30-kDa AgAPN2 truncation expressed in E. coli bound Cry11Ba with high affinity and inhibited Cry11Ba toxicity to An. gambiae larvae. Scanning block deletion analyses identified the region 336S - P420 essential for toxin binding and the inhibitory effect. My findings suggest that the glycosylation-independent interaction between AgAPN2 and Cry11Ba is important for toxicity induction. I also found a 28-kDa fragment of AgAPN2 that competitively bound Cry11Ba and synergized Cry11Ba toxicity. An in-frame deletion between 676I and W760 resulted in peptide's loss of toxicity enhancement without compromising binding affinity. In summary, my studies provide novel insights into the molecular mechanism underlying the toxin-receptor interaction and may lead to development of new bio-control strategies.