The horizontal transfer of the bacterium Wolbachia pipientis between invertebrate hosts hinges on the ability of Wolbachia to adapt to new intracellular environments. The experimental transfer of Wolbachia between distantly related host species often results in the loss of infection, presumably due to an inability of Wolbachia to adapt quickly to the new host. To examine the process of adaptation to a novel host, we transferred a life-shortening Wolbachia strain, wMelPop, from the fruit fly Drosophila melanogaster into a cell line derived from the mosquito Aedes albopictus. After long-term serial passage in this cell line, we transferred the mosquito-adapted wMelPop into cell lines derived from two other mosquito species, Aedes aegypti and Anopheles gambiae. After a prolonged period of serial passage in mosquito cell lines, wMelPop was reintroduced into its native host, D. melanogaster, by embryonic microinjection. The cell line-adapted wMelPop strains were characterized by a loss of infectivity when reintroduced into the original host, grew to decreased densities, and had reduced abilities to cause life-shortening infection and cytoplasmic incompatibility compared to the original strain. We interpret these shifts in phenotype as evidence for genetic adaptation to the mosquito intracellular environment. The use of cell lines to preadapt Wolbachia to novel hosts is suggested as a possible strategy to improve the success of transinfection in novel target insect species.