Eggs of an ichneumon fly, Nemeritis canescens, have been injected into endopterygote insects to which this parasite was alien. In larvae, pupae and adults of Tenebrio molitor (Coleoptera) almost all the eggs hatched; and within 24 h haemocytes enveloped the parasite larvae and melanin was deposited over their mouth and anus. The pattern of these reactions was the same in all cases and at all stages. When individuals of Tenebrio were given repeated injections at weekly intervals, they reacted to the last parasite exactly as to a first. Female individuals that had been repeatedly injected were allowed to develop and to lay eggs; when larvae reared from those eggs were injected they made the same characteristic reactions. No trace of increased tolerance or resistance on the part of Tenebrio was observed in either experiment. Larvae and pupae of Diataraxia oleracea (Lepidoptera) rapidly encapsulated parasites injected into them; adults made no visible reaction. Nemeritis larvae fed, grew and ecdysed in these adults, and only the premature death of the moth prevented the parasite's development. In feeding larvae, resting larvae and adults of Callipora erythrocephala (Diptera), the haemocytic and melanin reactions to injected parasites were both weak; in pupae the haemocytic reaction was weak but the melanin reaction strong. In the adult blowflies, parasite larvae lived as long as 7 days, but did not grow and seemed unable to use the food they had ingested. Observations recorded in this and the two preceding papers of the series are compared and discussed. The haemocytic reaction to Nemeritis, as observed in several orders of insects, is a true defence reaction and, when strong enough, leads to the death of the parasite. Deposition of melanin acts as a defence reaction only fortuitously, when the deposit is so sited as to prevent a vital activity, such as the hatching or the feeding, of the parasite. The reactions to Nemeritis of the different kinds of hosts so far investigated are distinguishable, but no constant difference has been observed that would serve to distinguish the reactions of exopterygote from those of endopterygote insects.