The endosymbiotic bacterium, Arsenophonus, is a secondary symbiont found in many insects, including psyllids, aphids and one triatomid. Our studies have shown the presence of Arsenophonus spp. in certain species of batflies. Interestingly, only the New World Streblidae seem to harbor this endosymbiont, while Old World Streblidae and Old and New World Nycteribiidae are not infected. This suggests a single infection event involving the common ancestor of the New World Streblidae. Using molecular data from the 16S, 23S, and RNA polymerase beta-subunit encoding gene [rpoB] from Arsenophonus and 18S, 28S, 16S, H3, co2, cytB from the host Streblidae, we elucidate the endosymbiotic and host phylogeny, and explore the question of host-endosymbiotic cospeciation in New World Streblidae.