Wolbachia is one of the most common endosymbionts found infecting arthropods. theory predicts symbionts like Wolbachia will be more common in species radiations, as host shift events occur with greatest frequency between closely related species. further, the presence of Wolbachia itself may engender reproductive isolation, and promote speciation of their hosts. Here we screened 178 individuals belonging to 30 species of the damselfly genera Nesobasis and Melanesobasis-species radiations endemic to the Fiji archipelago in the South Pacific-for Wolbachia, using multilocus sequence typing to characterize bacterial strains. incidence of Wolbachia was 71% in Nesobasis and 40% in Melanesobasis, and prevalence was also high, with an average of 88% in the Nesobasis species screened. We identified a total of 25 Wolbachia strains, belonging to supergroups A, B and f, with some epidemic strains present in multiple species. the occurrence of Wolbachia in both males and females, and the similar global prevalence found in both sexes rules out any strong effect of Wolbachia on the primary sex-ratio, but are compatible with the phenotype of cytoplasmic incompatibility. Nesobasis has higher species richness than most endemic island damselfly genera, and we discuss the potential for endosymbiont-mediated speciation within this group. Wolbachia is a genus of endosymbiotic α-Proteobacteria, which infects a wide range of hosts, including arthro-pods and filarial nematodes 1. Among the arthropoda, studies have estimated Wolbachia infects between 40% 2 and 52% 3 of species (including arachnids and crustaceans, but predominantly insects). A recent study on aquatic insects has estimated that 52% of these carry Wolbachia, a figure comparable to that found in terrestrial insects 4. In most cases however, a minority (<10%) of individuals within species are infected 5. Wolbachia are catego-rised as reproductive parasites, i.e. maternally inherited microorganisms that manipulate the reproduction of their hosts in ways that enhance the production or the survivorship of infected females, hence increasing their own fitness. Reproductive alterations induced by Wolbachia include the feminization of chromosomally male embryos, killing of male hosts during embryogenesis, induction of thelytokous parthenogenesis, and cytoplasmic incompatibility (CI). The latter prevents successful mating between infected males and uninfected females, or between individuals infected with different, incompatible Wolbachia strains 1. Beyond these reproductive manipulations , Wolbachia can also evolve mutualistic associations with their hosts, including facultative relationships that increase host fecundity or host survival/longevity, provide nutritional provisioning, or protect hosts against pathogenic attacks. In some cases hosts have evolved complete dependence upon Wolbachia 6,7. Vertical transmission of Wolbachia from mother to offspring occurs through the germline and somatic stem cell niches 8. On a longer evolutionary scale, vertical transmission can be identified by a shared phylogeny between the endosymbionts and their hosts. However, this pattern is usually not observed for Wolbachia, reflecting a process in which vertical transmission within species combines with occasional horizontal transmission between