When a host-regulative ectoparasite and a host-regulative endoparasite compete for the same host population (multiple parasitism) the former largely replaces the latter when the host’s density is high, the ectoparasite then being relatively numerous and the endoparasite scarce. However, after the ectoparasite has reduced the host population to a low density the converse is true, the endoparasite being more numerous. This difference is an effect of the endoparasite being able to operate at lower host densities; an essential adaptation if it is to survive competition with a host-regulative ectoparasitic population. The above conclusions are based (1) on the statistical analyses of numerous population samples of ectoparasitic and endoparasitic species of the family Aphelinidae (Chalcidoidea: Hymenoptera) imported into California while inhabiting host coccids (Diaspididae: Homoptera); (2) on field observations by the writer and the reports of others on the numerical status of such hosts and their specific parasites in gardens and orchards in various countries, those in which the species are indigenous as well as in those where they are exotic; (3) on the principles of multiple parasitism as elucidated by Harry S. Smith; and (4) on multiple parasitism by an ectoparasite and a host-specific endoparasite being characterized by the ectoparasite always destroying the endoparasite when both are attacking the same host individual and the endoparasite replacing the ectoparasite when populations of both are subsisting on a low density host population.