Fall armyworm larvae (FAW), which are serious pests in the southern United States, show retarded growth when they feed on insect-resistant maize inbreds Mp704 and Mp708. These maize genotypes are not only resistant to FAW, but to a number of other lepidopteran pests. In these genotypes, a unique, extracellular, 33-KDa cysteine protease (Mir1-CP) rapidly accumulates in the whorl in response to insect feeding. Initial morphological studies on larvae feeding on resistant maize plants over-expressing the cysteine protease showed severe damage in insect?s first line of defense, the peritrophic matrix (PM). But it is not known whether the cysteine protease has unprecedented effect on insect defense mechanisms. This study focuses on understanding the functional involvement of the cysteine protease (Mir1-CP) in a plant-herbivore defense mechanism. I used purified, recombinant 33-KDa cysteine protease (Mir1-CP) and its two mutated forms (Mut1 and Mut2) to determine their effects on the permeability of PMs from fall armyworm and other lepidopteran larvae. The purified Mir1-CP was also used to determine its minimal effective dosage on lepidopteran larval growth as well as to qualitatively determine their direct morphological effects on PM and gut regions of fall armyworm larvae. In vitro permeability studies demonstrated that the recombinant Mir1-CP directly permeabilized the PM and requires both cysteine at the active site and the terminal 25 amino acids to achieve complete permeabilization. Dose response study suggested that physiologically relevant concentrations of Mir1-CP in the maize whorl would be effective in controlling a broad range of lepidopteran pests. The study also suggested that stacking Mir1-CP and Bt-toxin (Bt-CryIIA) genes in transgenic plants could broaden the normal range of both Mir1-CP and Bt-toxin. Morphological studies using three different microscopic techniques showed damaged PM in larvae fed on Mir1-CP diet. These results suggest that by directly permeabilizing and damaging the PM, the Mir1-CP provides critical defense in host plants against lepidopteran pests.