Comparison of independently derived phylogenies for pocket gophers (Geomys) and their chewing lice (Geomydoecus) from Texas and Louisiana indicates a history of widespread cospeciation in this host-parasite assemblage. Inference of cospeciation is supported by statistical comparison of genetic-distance matrices for gophers and lice based on allozyme data. Although similar, host and parasite phylogenies are not identical; inconsistencies likely result from host-switching by the parasites, retention of ancestral taxa of parasites on recently evolved hosts, or poorly delineated taxonomic boundaries. The current disjunct distribution of Geomydoecus ewingi suggests that this chewing louse once parasitized the common ancestor of Geomys breviceps and G. attwateri. Combined protein and morphologic evidence suggests that the population of Geomydoecus ewingi hosted by G. breviceps breviceps in northeastern Louisiana may be a cryptic species of louse.