Xestia c-nigrum granulovirus (XcGV) was tested for its ability to increase Spodoptera litura nucleopolyhedrovirus (SlNPV) infection in larvae of S. litura (F.). The interaction of XcGV with peritrophic matrix and SlNPV in S. litura also was studied to account for the synergism. In dose-response bioassays with a constant XcGV concentration of 5-mg/ml capsules and SlNPV concentration that varied from 10³ to 10⁷ polyhedral inclusion bodies (PIB) per larva, XcGV increased the virulence of SlNPV infection in fifth instars of S. litura. The lethal concentration of 50% individuals (LC₅₀) of SlNPV combined with XcGV was 3.35 × 10⁵PIB/ml, which was significantly lower than that of SlNPV alone (2.17 × 10⁶). Compared with 10⁷ PIB/ml SlNPV alone, the lethal time of 50% individuals (LT₅₀) of 10⁷ PIB/ml SlNPV combined with XcGV was not significantly shortened. In addition, no significant improvement in the activity and killing speed of SlNPV progeny was noted after propagation with XcGV, indicating that native characters of SlNPV associated with viral potency were not altered by XcGV. Investigation via environmental scanning electronic microscopy showed that the peritrophic matrix (PM) of S. litura exposed to XcGV or XcGV enhancin, or the combination treatment, was markedly disrupted. The outer surface of the PM was loose, or ruptured, which potentially facilitated the passage of virions through the PM. Therefore, it is reasonable to conclude that the synergy between XcGV and SlNPV was closely associated with the disruption of the PM in S. litura.