Two new Nippostrongylinae, Malvinema yagoi sp. nov. and Stilestrongylus talaverensis sp. nov. are described from the intestine of the water rat Scapteromys aquaticus (Thomas) (Sigmodontinae) from the Province of Buenos Aires, Argentina. Malvinema yagoi sp. nov. is distinguished from the most closely related species M. frederici Digiani, Sutton et Durette-Desset, 2003, from the same host and locality, mainly by a caudal bursa with the right rays 3 and 6, and 4 and 5 of equivalent length, and the posterior end of the female ventrally folded. Stilestrongylus talaverensis sp. nov. is distinguished from the most closely related species S. manni Denke et Murua, 1977, a parasite of Akodon spp. from Chile, mainly by the presence of a short common trunk of rays 3 to 6 in the left lobe of the caudal bursa, the right ray 4 shorter than ray 5, the higher spicule length/body length ratio and the rotation of the posterior end of the female.