THE cytoplasmically inherited microorganism Wolbachia pipientis behaves like a sexually selected trait in its host, the flour beetle Tribolium confusum, enhancing male fertility at the expense of female fecundity. Here we show that infected females have fewer offspring than uninfected females but infected males have a large fertility advantage over uninfected males within multiply-inseminated infected or uninfected females. The male fertility effect accelerates the spread of the Wolbachia through the host population and expands the initial opportunity for hitch-hiking of host nuclear genes. Sperm competition in a host, mediated by endosymbionts, has not been previously described.