SOUTH AMERICAN METATHERIANS FROM THE BEGINING OF THE NEOGENE (EARLY MIOCENE, COLHUEHUAPIAN LAND-MAMMAL AGE). MICROBIOTHERIA AND POLYDOLOPIMORPHIA. We review the taxonomy of two South American orders of Metatheria (Mammalia) of Colhuehuapian age (early Miocene): Microbiotheria and Polydolopimorphia. First, we comment and discuss on the cusp homologies of the upper and lower molars of the Argyrolagoidea (Polydolopimorphia). Second, we offer results of a phylogenetic analysis of the Microbiotheriidae (Microbiotheria). Third, we analyze all Colhuehuapian Microbiotheria and Polydolopimorphia known up to date: Order Microbiotheria, Family Microbiotheriidae: Pachybiotherium acclinum Ameghino, Pachybiotherium sp., Clenia minuscula Ameghino, Oligobiotherium divisum Ameghino, Eomicrobiotherium mykerum sp. nov., Microbiotherium sp., and Microbiotheriidae indet.; Order Polydolopimorphia, Suborder Bonapartheriiformes, Superfamily Argyrolagoidea, Family Argyrolagidae: Proargyrolagus argentinus sp. nov. and Anargyrolagus primus Carlini, Pascual and Goin; Family Patagoniidae: Patagonia peregrina Pascual and Carlini; Argyrolagoidea indet. Finally, we add knowledge on the Colhuehuapian Didelphoidea, describing an indeterminate species referable to this superfamily, and comment on the identity of “ Micro biotherium” gutierrezi Simpson, recognizing the new combination Coona gutierrezi.