During a parasitological examination of 45 specimens of Acanthisthius brasilianus (Valenciennes, 1828) Jordan et Eigenmann, 1890, from waters off Mar del Plata, Argentina (38°08′S, 57°32′W), several specimens of cucullanid nematodes were collected from the intestines. A new species, Dichelyne (Cucullanellus) szidati n. sp., is described (prevalence 42.2%, mean intensity 2.7). The new species differs from its congeners inhabiting the southwestern Atlantic by the distribution pattern of caudal papillae, particularly the ninth pair, length of the body and spicules, and the position of the intestinal cecum (ventral), the excretory pore (posterior to esophagus), and the deirids (at the level of esophageal posterior end).