These studies show that Orgilus obscurator-purasitized Rhyacionia buoliana larvae initially gain weight in a manner similar to unparasitized larvae, but eventually attain a much smaller maximum size and then slowly decrease in weight with time. This decrease in weight is caused by the feeding of O. obscurator. The rate of increase in weight by O. obscurator increases with time during the second to fourth instars and decreases slightly during the fifth instar. During the fourth and fifth stages, the weight of O. obscurator changes from an initially minute proportion of the combined host–parasite weight to a significantly large proportion, eventually attaining ca. 80% of the combined weight. The feeding and activity of parasitized hosts remain high and continue for ca. 1 week longer than that of unparasitized hosts, resulting in a higher potential for damage to the host tree. Moulting of host larvae seems not to be affected by the presence of O. obscurator, although parasitized larvae remain in a "juvenile" state and do not develop testes. The implications of the size relationships of O. obscurator with its host on the activity of another parasite, Hyssopus thymus are discussed, and it is noted that parasitized hosts, on the average, needed fewer moults to complete their post-winter development than unparasitized hosts. This phenomenon is related to synchrony of the adult parasite with its host, and to host behaviour.