Wolbachiais a maternally inherited bacterial infection common in many insects. These bacteria cause cytoplasmic incompatibility, in which a cross between an infected male and an uninfected female is sterile. Infected females are always fertile, suggesting that an infected male produces a sterilizing product against which infected females are protected. This sterility trait is an evolutionary puzzle because it acts in males, but males never transmit the parasites. Previous work has suggested that the parasite gains by reducing the fecundity of uninfected females, thereby increasing the relative reproductive rate of infected females. This argument depends on kin selection effects: the parasite in the male does not reproduce, but can aid related parasites in neighbouring females. Formal population genetic models have failed to confirm the verbal kin selection models. Those models assumed pleiotropic gene action whereby incompatibility evolves as a correlated effect of other fitness components. A formal model presented here supports the original kin selection theories. This new model also suggests an explanation for observed variation in the degree of incompatibility amongWolbachiastrains isolated fromDrosophila simulans.