Seven species of helminths (6 nematodes and 1 cestode) were recovered from 38 plains vizcachas, Lagostomus maximus (Desmarest, 1817). Collections were made at 2 ranches, Los Valles Ranch (LVR) and El Estribo Ranch (EER), that are ∼300 km apart in La Pampa Province, Argentina. Prevalence and intensity of helminths from both LVR and EER were analyzed with respect to host gender, location, weight, season, and, where possible, colony. Between males and females there were no statistical differences in prevalence or intensity for the 7 helminths at each location. There was no difference in prevalence of Lagostonema ecasience between LVS and EER; however, the intensity was significantly higher at LVS (x̄ = 636) than at EER (x̄ = 6) (P = 0.003). Graphidioides affinis, Heteroxynema viscaciae, and Monoecocestus sp. are reported from the plains vizcacha for the first time. The most prevalent parasites were the nematodes L. ecasience (97%) and Viannella viscaciae (97%). An anterior shift in the distribution of V. viscaciae within the small intestine occurred with concurrent infections of Monoecocestus sp.