RODENTS AND MARSUPIALS AT THE PLEISTOCENE—HOLOCENE BOUNDARY IN CATAMARCA, ARGENTINA: EXTINCTIONS AND ENVIRONMENTAL EVOLUTION. A fossil small-mammal assemblage from the neighborhood Las Juntas, Catamarca, Argentina (28°06′36″S–65°53′54″W, 1562 m) was studied. A radiocarbon dating on a sample of long bones referred them to late Pleistocene (13.4–13.0 cal ky). The assemblage includes twelve species, dominated by the sigmodontine rodents Reithrodon auritus (Fischer) and Phyllotis sp., followed by Akodon spegazzinii Thomas and Andinomys edax Thomas. The presence of the caviomorph Abrocoma cinerea Thomas, and the sigmodontines Akodon simulator Thoma s and Tafimys powelli Ortiz, Pardiñas and Steppan is remarkable. A Didelphimorphia, Thylamys sp., is also present. The surface of the bones and teeth shows microscopical evidences of corrosion, indicating that the sample was generated by the predatory activity of owls. The identified species correspond to elements of high-altitude open environments, a mixture of cloud highland-grasslands, high-Andean grasslands, and scarce elements of upper montane forest. Most of them are currently sympatric at an altitude of approximately 2500 m in the same mountain range, in a colder and drier environment than that recorded at the studied site. It is suggested that paleoenvironmental conditions during deposition of the assemblage were colder and drier. It is not possible to extrapolate these data to other areas because of the great orographic and environmental variation in the region, which determines different small-mammal assemblages across relatively reduced geographic areas.