A study of type material of Bucephalopsis rioplatensis Szidat, 1970 from intestine of the flatfish, Catathyridium jenynsii (Günther, 1862) (as Achirus trichospilus Berg, 1895) from the Rio de la Plata, Buenos Aires Province, Argentina, revealed that the ranges and morphological variations originally reported by Szidat (1970) and ascribed to age of specimens, appear to be caused by pressure during fixation; although some of the variation reported may have been a result of difference in age. This study redescribes the species and placed it in the genus Prosorhynchoides Dollfus, 1929 as P. rioplatensis (Szidat, 1970) comb. nov.