Abstract Thylamys pallidior (O. Thomas, 1902) is a didelphid marsupial commonly called the white-bellied fat-tailed mouse opossum. A small mouse opossum, with tricolored pelage and a white venter, it has a seasonally incrassated tail, where fat deposits, primarily at the tail base, function in food storage. It is the most widely distributed species in the genus occurring from southwestern Peru and northern Chile southward along the eastern slopes of the Andes to southern Argentina. This species is found primarily in arid and semiarid habitats at elevations from sea level to about 4,500 m. It is not of special conservation status but may be uncommon in some areas.