Antarctodolops dailyi, a new genus and species of Polydolopidae, a group of marsupial mammals heretofore known only from strata of early Tertiary age in South America, is described from rocks about 40 Ma old in the Antarctic Peninsula. This is the first fossil or Recent land mammal to be found in Antarctica. A phylogenetic analysis of the new find is presented, along with consideration of its implications as to past continental land connections between South America, Antarctica, and Australia. This analysis is set in the perspective of the Late Cretaceous and early Tertiary positions of the southern continents.--Modified journal abstract.