First record of Mastophorus muris (Gmelin, 1790) (Nematoda: Spiruroidea) from a wild host in South America.

Mastophorus muris (Gmelin, 1790) (Nematoda: Spiruroidea) is reported parasitizing the grey leaf-eared mouse Graomys griseoflavus (Waterhouse, 1837) (Rodentia: Sigmodontinae) from the province of La Pampa, Argentina. The distinct position of Mastophorus (Spirocercidae: Mastophorinae) and Protospirura (Spiruridae), sometimes still confused, is again confirmed. The pattern of pseudolabial teeth (a large central tooth with smaller teeth on each side), which seems to be rather stable in all known descriptions, is here confirmed with the aid of scanning electron microscopy. The finding represents the first record of the species in Argentina, but also from a wild host in South America. This indicates an expansion of the distribution range of the species, which, in the subcontinent, was hitherto restricted to domestic rodents.