We previously suggested that Armigeres subalbatus (Coquillett) prophenoloxidase III (As-pro-PO III) might be associated with morphogenesis of larvae and pupae. Because PO and its activation system are present in the insect cuticle, and cuticle formation is a major event during pupal morphogenesis, we used ultrastructural analysis to examine the effects of As-pro-PO III knockdown on the formation of pupal and adult cuticle. Inoculation of As-pro-PO III dsRNA resulted in the incomplete formation of nascent pupal endocuticle and pharate adult cuticle, i.e., significantly fewer cuticular lamellae were deposited, the helicoidal pattern of chitin microfibrils was disorganized, and numerous electron-lucent spaces were present in the cuticular protein matrix. Similar disruptions were observed in the cuticle of adults derived from As-pro-PO III dsRNA—inoculated pupae. It has long been suggested that the quinines, generated by PO-catalyzed oxidation reactions, function as cross-linking agents; therefore, it seems reasonable to suggest that the loss of As-pro-PO III—mediated protein-protein linkages causes morphological abnormalities in the protein matrix. Our findings suggest that As-pro-PO III plays a role in cuticle formation in mosquitoes, a novel function for phenol-oxidizing enzymes.