Eggs of varying degrees of genetic foreigness were implanted into the hemocoel of an insect larvae (Pimpla instigator, Hymenoptera). For xenogenic implants, eggs of Drosophila melanogaster (Diptera) and eggs of Apechthis compuctor (Hymenoptera) were employed. Pimpla eggs of stock culture and Pimpla eggs of genetically defined pure strains were used for allogenic implants and isogenic implants, respectively. The host response that has been studied is that of encapsulation; it was possible to examine quantitatively the effect of genetic relationship on the degree of cellular reaction, i.e., thickness of capsule. Encapsulation occurred with increasing intensity as the degree of genetic difference increased, indicating that Pimpla larvae were able to discriminate not only between self and not-self, but also between different degrees of not-self.