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An independent species of cestode of the genus Diphyllobothrium Cobbold, 1858, known only from the Pacific walrus, Odobenus rosmarus divergens Illiger, has been incorrectly designated Diphyllobothrium cordatum (Leuckart, 1863) in the literature since 1955. That cestode is described in this study as Diphyllobothrium fayi n. sp. on the basis of specimens obtained from walruses from the northern Bering Sea and the Arctic Ocean. It is distinguished readily from D. cordatum by its much larger strobila of different form and by a range of morphological characteristics of taxonomic significance in the reproductive organs. Comparisons were made with specimens of D. cordatum from the bearded seal, Erignathus barbatus (Erxleben), its typical host. Diphyllobothrium fayi has been recorded only from the Pacific walrus. Although that mammal feeds mainly on mollusks and other benthic invertebrates, records of cestodes from it suggest that fishes potentially harboring plerocercoids are frequently consumed.