Diclidophora nezumiae sp. n. is described from the gills of the rat-tail fish Nezumia bairdii (Goode and Bean, 1877) taken from the environs of Hudson Submarine Canyon in the northwest Atlantic. The host-parasite relationships were studied in the host population. The new species is most similar to small species of Diclidophora having short bodies that taper to maximum width at the level of the first pair of clamps. It may be differentiated from other species by the following: clamps wider than long, noticeably decreasing in size posteriorly; lamellate extension of sclerite b does not fuse with sclerite c1; unsclerotized diaphragm; a relatively small clamp sucker; 10-13 cirrus hooks; 18-30 intercecal, postovarian testes; unlobed seminal receptacle; filamented eggs; and body dimensions. Of 378 N. bairdii specimens examined, 106 (28%) were infected with 1-21 D. nezumiae per host. The parasite occurred most frequently on filaments of the first gill arch. Infected fish ranged from 61-428 mm in total length. Th...