Neoechinorhynchus (Neoechinorhynchus) buckneri n. sp. is described from the blacktail redhorse Moxostoma poecilurum (Jordon) (Catostomidae) in the Tchoutacabouffa River, Mississippi, U.S.A. It is the sixteenth species of Neoechinorhynchus Stiles and Hassall, 1905, with unequal lemnisci, and the sixth in a subgroup with dorsal body wall markedly thicker than ventral. Another species of the latter group, Neoechinorhynchus (Hebesoma) carinatus Buckner and Buckner, 1993, was also collected in the Tchoutacabuffa River, Mississippi, and tributaries of 2 other streams in Louisiana, U.S.A., from the sharpfin chubsucker, Erimyzon tenuis (Agassiz). All 3 streams from the 2 adjacent states discharge into the Gulf of Mexico. Most of these 16 species occur in North American freshwater fishes. Females of N. buckneri n. sp. are twice as long as males and their dorsal body wall is 4 times as thick as the ventral body wall, compared with 2.5 times as thick in males. All other shared structures are of similar size in both sexes. Proboscis armature is unremarkable, with hook circles being evenly spaced and hook sizes in each circle of similar size, but slightly decreasing in size posteriorly from 38–42 µm long anteriorly to 33–34 µm long at middle to 25 µm long posteriorly in both sexes. It is further distinguished from the group of 16 other species of Neoechinorhynchus with unequal lemnisci and body wall thickness, by having moderately but not extremely diverse leminisci, noncontiguous testes, and posterior testis distant from cement gland. A key distinguishing N. buckneri n. sp. from other species with different dorsoventral body wall thickness is provided. Scanning electron microscopy studies show the porous cuticular surface and the curling of the posterior end of breeding females caused by the contraction of the paired paravaginal muscles, among other features.