Marsupial mammals are herein described from the Kaiparowits Formation of southcentral Utah, of probable Judithian (Campanian, Late Cretaceous) age. New materials of Iqualadelphis lactea, previously described from the upper Milk River Formation of Alberta, support the hypothesis that the species is the most primitive known marsupial. A new species, intermediate in morphology between the Aquilan “Alphadon” creber and the Judithian to Lancian “A” lulli, is described, and the three are removed to a new genus, Protalphadon. Two new species closely similar to Judithian “Alphadon” russelli and “A” praesagus, respectively, are described; these, together with the Lancian “A” rhaister share derived morphology not found in other North American Cretaceous peradectids, and are placed in the new genus Turgidodon. New materials of the Judithian Alphadon halleyi and A. sahnii support the distinction of these closely similar species. Knowledge of the diminutive Alphadon attaragos, described on the basis of an isolated lower molar, is improved by molars representing several upper and lower loci. Marsupials of the Kaiparowits Formation are most closely similar to those previously reported from the Judithian, although the relative primitiveness of several Utah species and the presence of a taxon otherwise restricted to the Aquilan suggests that assemblages from the lower Kaiparowits Formation, at least, may be somewhat older than typical Judithian faunas. It is also likely that the distinctiveness of the Kaiparowits taxa from those represented in more northerly faunas is, in part, due to Zoogeographic differentiation among North American Cretaceous terrestrial vertebrates. Pediomyid and stagodontid marsupials, for instance, are important elements of most known assemblages from the north-central Rocky Mountain region; these are rare or absent in the Kaiparowits Formation.