Heterakis spumosa and Syphacia muris are among the most common species parasitizing mammals. Although these nematodes have been reported from many hosts on several continents, in South America, they have only been recovered from the cecum of Rattus norvegicus from Brazil and Peru. This study provides morphological details of H. spumosa and S. muris. Moreover, this is the first record of these species in Argentina. Heterakis spumosa and S. muris were collected from specimens of R. norvegicus from Partido de Exaltación de la Cruz, Buenos Aires Province, Argentina. This note presents relevant features observed with scanning electron microscopy (SEM) for H. spumosa, such as the presence of 4 papillae on the lips, the lateral alae forming a cuticular fold inside a groove that extends to the posterior end, double papillae on the tip of tail of the male, vulva with 5 cuticular processes, and 2 pair of sessile papillae on the female tail. Syphacia muris has a porous badge posterior to the amphids, and the excretory pore is located in an oval depression.