Rates of virus-induced mortality and values of the LC50 and LT50 showed that, in general, parasitism by Microptitis rufiventris Kok. increased the susceptibility of the Egyptian cotton leafworm, Spodoptera littoralis second-instar larvae to its nuclear polyhedrosis virus (SLNPV). By comparing the LC50 values, data revealed that virus-infected parasitized larvae were ca. 3.5 and 4.8 times more susceptible to SLNPV than virus-infected unparasitized ones at 8 and 12 days post-infection, respectively. Also, in virus-infected unparasitized larvae, the time required to attain a 50% mortality was significantly longer than that in virus-infected parasitized ones.