The American swallow bug (Oeciacus vicarius) is an important ectoparasite of cliff swallows (Petrochelidon pyrrhonota) and is known to harbor several types of arbovirus. A recent study in northeast Texas suggested that O. vicarius might occur in barn swallows (Hirundo rustica) at rates much higher than previously thought. The purpose of this study was to determine the extent to which barn swallows in northeast Texas are parasitized by O. vicarius and how well this parasite is adapting to this novel host. A sample of 498 nests at 54 colonies was inspected for O. vicarius. Forty colonies (74.1%) were infected, while 310 nests (62.2%) were infected. Large colonies were more likely to be infected than small colonies. Colonies that also contained cliff swallows were more likely to be infected than colonies without cliff swallows. Infection levels in barn swallow nests were comparable to those reported for cliff swallows, though age and sex class ratios differed. Demographic changes among swallow species, including range expansions, increased colony sizes, and more frequent interspecific nesting associations have likely facilitated the movement of O. vicarius from the cliff swallow into a novel host, the barn swallow.