We discovered a novel symbiotic bacterium of the genus Rickettsiella in European natural populations of the pea aphid Acyrthosiphon pisum, and demonstrated that the symbiont induces a drastic color change of the host aphids: originally red insects turned into green when infected. Body color is an ecologically important trait, often involved in species recognition, sexual selection, mimicry, aposematism, and crypsis. However, there has been no report on such a phenomenon that the important biological trait, body color, is drastically changed by a symbiotic microorganism. This finding provides a new viewpoint to the ecology and adaptation of insects and other organisms in general.