Aphids commonly harbour facultative bacterial endosymbionts and may benefit from their presence through increased resistance to parasitoids. This has been demonstrated for Hamiltonella defensa and Serratia symbiotica, while a third common endosymbiont, Regiella insecticola, did not provide such protection. However, this symbiont was recently detected in a highly resistant clone of the peach-potato aphid, Myzus persicae, from Australia. To test if resistance was indeed conferred by the endosymbiont, we eliminated it from this clone with antibiotics, and we transferred it to two other clones of the same and one clone of a different aphid species (Aphis fabae). Exposing these lines to the parasitoid Aphidius colemani showed clearly that unlike other strains of this bacterium, this specific isolate of R. insecticola provides strong protection against parasitic wasps, suggesting that the ability to protect their host against natural enemies may evolve readily in multiple species of endosymbiotic bacteria.