We present a systematic revision of the mouse opossums of northwestern Arge
ntina; included are three genera and eight species, Gracilinanus (new for t
he region), Micoureus, and Thylamys. Two subspecies of Thylamys are elevate
d to species. The distribution of each of the taxa was determined using spe
cimen records. External and cranial descriptions, measurements, and comment
s on the taxonomy and natural history of each species are given.