The influence of parasitism by Glyptapanteles liparidis (Bouche) on the development, consumption, growth and food utilization from the third to the fifth instar of its host Lymantria dispar L. was studied on three different host types, each characterized by the stage in which they were parasitized. The first host type was parasitized during premoult into the 2nd instar, the second during premoult into the 3rd instar and the third during the middle of the 3rd instar. Depending on the relationship between the host's size and the volume of the parasitoid complex inside the host, different developmental patterns of the various host types were found. From the majority of early-parasitized hosts, the parasitoid larvae emerged during the hosts' fourth instar (90%), while parasitoids emerged mainly during the fifth instar (77% and 81%) of both later-parasitized host types. Hosts which were heavily parasitized during their premoult stage, avoided obvious nutritional constraints by inserting a supernumerary host instar. Only from one lightly parasitized host type did parasitoid larvae emerge, after developing through only two host instars. Third-midinstar-parasitized hosts had a supenumerary instar only when they contained a relatively low number of parasitoids compared with those from which the parasitoids emerged during the fifth instar. Unparasitized gypsy moth larvae also differed in their developmental pattern, as (78% of males and 72% of females) pupated after the 5th and 6th instars, respectively. Differences in growth and development time permitted the differentiation between sexes, before larvae moulted to the fifth instar. Parasitism most influenced host nutrition in early-parasitized hosts, as they showed increased rates of consumption and growth during the 3rd, penultimate host instar. Higher biomass gains during their final fourth instar of these early-parasitized hosts were due to prolongation of both development and the feeding period, which resulted in a relative consumption and growth rate significantly lower than in unparasitized female larvae and later-parasitized larvae. Growth and consumption rates during the 3rd and 4th instar of later-parasitized host types did not differ from female control larvae. In their final 5th instar these host types did not prolong their development or feeding period, but displayed a reduction in growth rate similar to early-parasitized hosts. The approximate digestibility (AD-value) and conversion efficiency of ingested and digested food (ECI- and ECD-values) was not influenced by parasitism. No correlation was found between the number of parasitoid larvae per host and the duration of the endoparasitic development among most individuals of the three host types. The number of parasitoids per host, however, contributed markedly to the final gross weight, consumption, growth rate and biomass gained during the final host instar.