The acuarioid nematode Paracuaria adunca (Creplin, 1846) was found in 7 of 13 (54%) Larus dominicanus (Aves: Laridae) examined from several locations along the Argentine coast. It was only present in specimens from Patagonia; the intensity of infection ranged from 1 to 14 (mean = 5.4). This is the first record of P. adunca from South America and as a parasite of L. dominicanus. Using both light and scanning electron microscopy (SEM), the complexity of the left spicule and the number and arrangement of male caudal papillae were observed. Although P. adunca is parasitic in several unrelated bird families and has a wide geographical distribution, the dimensions of our specimens fully agree with previous reports. The genus Paracuaria was characterized by the presence of rudimentary cordons; thus, it was considered the most primitive acuarioid. The cordons appear as finger-like structures within shallow grooves held tightly against the surface and extend posteriorly from the ends of the mouth and for this reason cannot be easily seen under SEM.