Approximately 1,350 specimens of an undescribed species of Austrodiplostomum (Diplostomidae: Diplostominae) were collected from 10 double-crested cormorants, Phalacrocorax auritus (Phalacrocoracidae), from the Galveston, Texas, U.S.A., area. Austrodiplostomum compactum is synonymized with Austrodiplostomum mordax, the type species in the genus, leaving A. mordax as the only species remaining. Austrodiplostomum ostrowskiae n. sp. can be distinguished from A. mordax by having a small but distinct hindbody, shorter ceca, a uterus and vitelline fields that extend into the tribocytic organ, smaller eggs, and an anterior testis that is about the same width as the posterior testis in A. ostrowskiae n. sp. Five specimens of the neotropical cormorant, Phalacrocorax brasilianus, were also examined from the Galveston, Texas, U.S.A., area during the study, but this host species was not found to be infected with a species of Austrodiplostomum.