Partial sequences of the 28S rDNA (ribosomal gene) were obtained from a total of 11 populations of 5 species (in 3 genera) of North American corallobothriine proteocephalideans. These included Corallobothrium fimbriatum (3 populations), Corallobothrium parafimbriatum (1 population), Corallotaenia minutia (1 population), Megathylacoides giganteum (2 populations), and Megathylacoides lamothei (4 populations). These sequences were used in a phylogenetic analysis to test the monophyly of Corallobothriinae and to investigate the interrelationships of the North American species. The results indicate that Corallobothriinae, as conventionally understood, is not monophyletic and that only the North American corallobothriines, parasites of ictalurid catfishes, form a monophyletic group. Corallobothrium parafimbriatum is sister taxon to a clade that includes Corallotaenia intermedia and C. minutia and not to its congener C. fimbriatum. Also, M. giganteum from Mexico appears to be more closely related to M. lamothei than to its conspecific in Canada. This and the amount of sequence divergence indicate possible cryptic speciation in its endemic host, the Lerma catfish, Ictalurus dugesi.