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The extant plains vizcacha, Lagostomus maximus, is described from the Late Pleistocene (Dolores Formation) of Uruguay based on an almost complete articulated skeleton. It is compared with the nominally extinct Pleistocene species of the genus. An AMS C-14 taxon-age is determined for L. maximus at 11 879 95 years BP (cal. BP 13 898-13 941). Lagostomus maximus is absent from modern mammal communities in Uruguay, and no Holocene evidence is available. Because L. maximus exhibits remarkable sexual and ontogenetic dimorphism, we examined a range of juvenile, sub-adult and adult male and female specimens. It is not possible to differentiate the articulated Pleistocene fossil from sub-adult specimens of L. maximus based on this sample. Moreover, the skull characteristics, including a broad vertical ramus of the zygomatic arch, semi-circular temporal crest and short robust sagittal crest, most closely resemble extant female individuals. Molar size is non-predictive for ontogenetic stage or body mass because it reaches stability during early adulthood. Other fragmentary skull remains are herein assigned to L. sp. cf. L. maximus. Lagostomus cavifrons from the Pleistocene of Argentina is considered synonymous with L. maximus. Its character states concur with the range of intraspecific variability and it has been established on a young adult or adult male individual. Likewise, the other Argentinean Pleistocene species based on incomplete mandibles, L. angustidens, L. striatus, L. heterogenidens, L. egenus, L. minimus and L. debilis, concur with ontogenetic morphs and are here assigned to Lagostomus sp. The ecological preferences of extant L. maximus infer open arid or semi-arid landscapes for the latest Pleistocene of southern Uruguay. This hypothesis is reinforced by the coeval presence of Microcavia, Galea and small camelids in the same strata. The Last Glacial Maximum likely promoted this environmental setting. Latest Pleistocene or early Holocene climatic change might have facilitated local extinctions and/or range shifts among this mammal fauna.