Natural enemies are important ecological and evolutionary forces, and heritable variation in resistance to enemies is a prerequisite for adaptive responses of populations. Such variation in resistance has been previously documented for pea aphids (Acyrthosiphon pisum) attacked by the parasitoid wasp Aphidius ervi. Although the variation was presumed to reflect genotypic differences among the aphids, another potential source of resistance to A. ervi is infection by the facultative bacterial symbiont Hamiltonelia defensa. Here, we explored whether variation among symbiont isolates underlies variation among A. pisum clones in resistance to A. ervi. Although maternally transmitted, H. defensa is sometimes horizontally transferred in nature and can be experimentally established in clonal aphid lineages. We established five H. defensa isolates in a common A. pisum genetic background. All of the five isolates tested, including one originating from another aphid species, conferred resistance. Furthermore, isolates varied in levels of resistance conferred, ranging from 19% to nearly 100% resistance. In contrast, a single H. defensa isolate established in five different aphid clones conferred similar levels of resistance; that is, host genotype did not influence resistance level. These results indicate that symbiont-mediated resistance to parasitism is a general phenomenon in A. pisum and that, at least for the isolates and genotypes considered, it is the symbiont isolate that determines the level of resistance, not aphid genotype or any interaction between isolate and genotype. Thus, acquisition of a heritable symbiont appears to be a major mode of adaptation to natural enemy pressure in these insects. Aphidius ervi (Aphidiidae) : Parasite Acyrthosiphon pisum (Aphidoidea) : Host Genetics; Evolution; Associations; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts Acyrthosiphon pisum; Phenotype; Resistance phenotypes; Genotype; Variation; Resistance variation; Mutualism; Hamiltonella defensa (bacterium); Hymenopteran parasites; Variation in resistance to parasitism; role of bacterial symbionts vs; host genotype; Aphidius ervi Aphidius ervi; Hemipteran hosts; Variation in resistance to parasitism; role of host-associated bacterial symbionts vs. genotype; Acyrthosiphon pisum none