We provide the first report of nematode parasites from the introduced American bullfrog (Lithobates catesbeianus) in San Juan Province, Argentina. Sixteen individuals (7 adult females, 6 adult males, and 3 juveniles) of L. catesbeianus were studied. Thirteen (81.3%) of the frogs harbored 2 species of nematodes: larvae of Contracaecum sp. (Nematoda, Anisakidae) in the serosal surface of the stomach wall and at the gastroduodenal junction, and adults of Falcaustra sanjuanensis (Nematoda, Kathlaniidae) in the large intestine. Previous studies of introduced populations of American bullfrogs in Argentina have focused on population structure, trophic ecology, and their impacts on native amphibian and other vertebrate populations. More studies of their parasites need to be conducted in South America in general, and in Argentina specifically, in order to assess the extent to which bullfrogs either introduce new parasites or are infected by native parasites and to determine the potential impacts of nonnative parasites on native frog species.