Endosymbiont interactions with hosts have important effects on fitness, including the fitness of many pest and beneficial species. Among these interactions, facultative endosymbiotic bacteria can protect aphids from parasitoids. Aphis craccivora and Acyrthosiphon pisum can harbor the symbiotic bacteria Hamiltonella defensa and its bacteriophage APSE. Infection by H. defensa defends these aphids against some but not all parasitoid species in the hymenopteran family Braconidae. Here, we report results on the effect of H. defensa on parasitism of these aphids by species in the other major lineage of aphid parasitoids, Aphelinus species in the family Aphelinidae. Parasitism of aphids infected with H. defensa /APSE by two Aphelinus species did not differ from that of uninfected aphids. While Aphelinus atriplicis showed no difference in fitness components between infected and uninfected aphids, Aphelinus glycinis actually produced more adult progeny and larger female progeny on infected than on uninfected aphids. Aphelinus glycinis may increase host quality for itself by changing the titer of nutritional versus protective bacteria in such a way that aphids infected with H. defensa can be made more suitable for parasitoid development than uninfected aphids. Our results and reasoning suggest that these Aphelinus species may be less prone to harm by H. defensa /APSE that affect eggs because they have anhydropic, heavily chorionated eggs, which may not absorb toxins during embryogenesis. See the included methods-DefensiveAphidSymbiont.txt file for more information on the methods and design structure of this study.