Larvae ofPieris rapaeL., when parasitized by larvae ofApanteles glomeratusL., have a 57% greater dry biomass at the beginning of the 5th instar and accumulate energy and nitrogen into their biomass at significantly higher rates (34 and 32%, respectively) during this instar than normal larvae. This occurs because parasitized larvae convert 17% more of their assimilated energy (net growth efficiency) and of their ingested nitrogen (nitrogen utilization efficiency) into biomass; the difference in the efficiency of energy assimilation between parasitized and normal larvae is slight. Also, parasitized larvae consume energy and nitrogen at absolute rates which are 14% faster than normal larvae.In spite of a 14% lower relative food consumption rate (mg dry food ingested/mg dry larval biomass × day), parasitized larvae have a relative growth rate (mg dry biomass gained/mg dry larval biomass × day) similar to normal larvae. This occurs because parasitized larvae have a 21% lower relative metabolic rate (cal oxidized/mg dry larval biomass × day). This lower relative metabolic rate also contributes to the higher net growth efficiency of the parasitized larvae.Host compensation for a supposed energy and nutrient drain caused by the feeding of the parasites may be involved in these changes but passive effects due to the presence of the parasites in the host and active influence on host physiology by the parasites are probably foremost.Food utilization budgets for normal and parasitized 5th-instarP. rapaelarvae are presented. During their life within a host larva, a mean of 28A. glomeratuslarvae/host consume ca. 50–60% of the energy and 60–70% of the nitrogen assimilated by the host without reducing energy or nutrient intake rates or laboratory survival of the host.