:A bacterial endosymbiont was previously observed in the green alga Volvox carteri strain UTEX 2180 using transmission electron microscopy, although its phylogenetic status was unknown. Here, we identified the bacterial endosymbiont based on analyses of the 16S ribosomal RNA (rRNA) gene. The endosymbiont of V. carteri belongs to the “hydra group” characterized by non-arthropod hosts, within the eubacterial family Rickettsiaceae (Rickettsiales; Alphaproteobacteria). In the hydra group, the endosymbiont formed a clade with the endosymbionts of the volvocaleans Carteria cerasiformis and Pleodorina japonica and the ciliate Diophrys appendiculata but was separated from the endosymbionts of marine green macroalgae (Bryopsis spp.). Fluorescence in situ hybridization using a probe specific for the 16S rRNA of the V. carteri endosymbiont confirmed our molecular identification and the distribution of the endosymbiont within the host cytoplasm. Rickettsial endosymbionts were not detected in eight other strains of V. carteri by 4′-6-diamidino-2-phenylidole (DAPI) staining and genomic PCR.