SUMMARYHost nutrition plays a major role in the nutritional physiology of Trichoplusia ni parasitized by the hymenopterous insect parasite, Hyposoter exiguae. Severely reduced growth rate characterized the host association throughout the 4th developmental stadium. This effect of parasitization, however, was indirect and growth depression of parasitized larvae was entirely accounted for by the accompanying decreased rate of food consumption. Parallel-fed larvae, that is, unparasitized larvae feeding on nutrients at the same rate as observed in ad libitum-fed parasitized individuals, displayed lower rates of growth than parasitized larvae and the latter had higher rates of assimilation. Parasitization, therefore, directly resulted in an increased rate of assimilation over that observed in uninfected insects after accounting for the effects of altered food consumption. Similarly, differences in the pattern of response to decreased dietary protein levels between parasitized and unparasitized insects could be explained on the basis of differences in their rates of food consumption