Ascaridoid nematodes occurring in South American mammals are divided into categories based on their possible origin. The affinities are discussed of five species so far known only from the Neotropical Region. Toxocara alienata (Rudolphi 1819) is reported from Nasua rufa socialis, Procyon cancrivorus, and Tayassus torquatus. The specimens from T. torquatus are described and found most closely to resemble Toxocara mackerrasae from south-east Asian and Austrialian rodents. Anisakis insignis from Inia geoffrensis is transferred back to Peritrachelius Diesing, 1851, on account of the structure of the lips and spicules. P. insignis is shown to exhibit remarkable convergence of lip structure with Lagochilascaris turgida from Didelphis marsupialis. Galeiceps longispiculum (Freitas & Lent, 1941) from Pteronura brasiliensis is confirmed as a species distinct from G. cucullus (Linstow, 1899) and G. spinicollis (Baylis, 1923), but G. simiae (Mosgovoy, 1951) is considered to be a synonym of G. spinicollis. An error in the host record of G. spinicollis is corrected from Cercopithecus leucampyx kandti to Lutra maculicollis kivuana. Ascaris dasypodina Baylis, 1922 from armadillos, including Cabassous unicinctus and Tolypeutes matacos, is redescribed and placed in a new genus Bairdascaris. The question is raised as to whether some species in Lagochilascaris, Galeiceps, and Toxocara may have crossed directly by sea from Africa to South America, rather than entering via North America.