Many species harbor the incompatibility-inducing microbe Wolbachia, a maternally inherited endoparasite that causes reduced egg hatch in crosses between infected males and uninfected females. Infected females are immune to this effect, which gives them a relative fitness advantage that results in the spread of the infection. The strength of incompatibility, fitness deficits associated with the infection, and transmission rate from mother to offspring largely determine the rate and extent of spread of Wolbachia in a population. We transferred Wolbachia from Drosophila simulans to Drosophila serrata, a novel host, and compared parameter estimates with those from three naturally occurring Drosophila-Wolbachia associations believed to be of different ages. Transfected D. serrata showed strong incompatibility, low transmission efficiency, and an associated fitness deficit, and they would probably be unable to spread in nature. The comparisons generally supported the predicted evolution of a host-Wolbachia association. The parameters peculiar to any given host-Wolbachia association may determine whether the microbial strain can spread in that host.