Several expressed sequence tags (ESTs) with homology to chitin deacetylase-like protein (CDA) were selected from a group of Helicoverpa armigera genes whose expression changed after infection with H. armigera single nucleopolyhedrovirus (HearNPV). Some of these ESTs coded for a midgut protein containing a chitin deacetylase domain (CDAD). The expressed protein, HaCDA5a, did not show chitin deacetylase activity, but it showed a strong affinity for binding to chitin. Sequence analysis showed the lack of any chitin binding domain, described for all currently known peritrophic membrane (PM) proteins. HaCDA5a has previously been detected in the H. armigera PM. Such localization, together with its downregulation after pathogen infection, led us to hypothesize that this protein might be responsible for the homeostasis of the PM structure and that, by reduction of its expression, the insect may reduce PM permeability, decreasing the entrance of baculovirus. To test this hypothesis, we constructed a recombinant nucleopolyhedrovirus to express HaCDA5a in insect cells and tested its influence on PM permeability as well as the influence of HaCDA5a expression on the performance of the baculovirus. The experiments showed that HaCDA5a increased PM permeability, in a concentration-dependent manner. Bioassays on Spodoptera frugiperda and Spodoptera exigua larvae revealed that NPV expressing HaCDA5a was more infective than its parental virus. However, no difference in virulence was observed when the viruses were injected intrahemocoelically. These findings support the downregulation of a midgut-specific CDA-like protein as a possible mechanism used by H. armigera to reduce susceptibility to baculovirus by decreasing PM permeability.