Endosymbiotic microorganisms affect the host ecology and physiology in a variety of ways. Members of the genus Wolbachia are the most widespread endosymbionts in insects, which are found intracellularly and transmitted maternally. In general, Wolbachia inhabit almost all tissues of the host insect, like mitochondria, and reduce the host fitness by manipulating the host reproduction. The reproductive phenotypes caused by the symbionts, such as cytoplasmic incompatibility, male-killing, etc., profoundly influence the host insect biology, potentially contributing to the insect diversity. Recently, I discovered a novel type of Wolbachia-insect association in the bedbug Cimex lectularius, which is known as a blood-sucking hygienic pest. Wolbachia infection in this insect was restricted to a specialized organ for housing symbionts called bacteiome. Elimination of the symbiont resulted in high mortality and retarded growth of the host, seriously reducing the host fitness. When the Wolbachia-eliminated insects were supplied with B vitamins, which are insufficiently contained in vertebrate blood, the host fitness was dramatically restored. These results indicate that the Wolbachia endosymbiont is an obligate mutualist supplying essential B vitamins for the bedbug host, implying that the Wolbachia might have played important roles in the evolution of blood feeding habit in bedbugs.