New discoveries in the latest Eoceneaged Florissant Formation in Florissant Fossil Beds National Monument, Colorado more than double the known diversity of fossil mammals from this rock unit, and provide a window into a diverse micromammal fauna that lived near Lake Florissant. Taxa first recorded here from the Florissant Formation include the lipotyphlans Centetodon magnus and Domnina cf. D. thompsoni, the lagomorph Megalagus brachyodon, and rodents Pelycomys, Eutypomys parvus, Ischyromys cf. I. typus, I. cf. I. doruglassi, and Adjidaumo minimus, all represented by dental material. We also describe isolated teeth of the artiodactyl Leptomeryx and the brontothere Megacerops. The mammalian fauna corroborates the Chadronian (latest Eocene) age of the Florissant Formation determined by others; Megalagus brachyodon suggests an age no older than Middle Chadronian. Several taxa reported here represent geographic range extensions. Eutypomys parvus is extended southeastward from southwestern Montana; ranges of Domnina thompsoni, Megalagus brachyodon, and Ischyromys douglassi are extended southwestward from western Nebraska; and ranges of Pelycomys, Ischyromys typus, and Adjidaumo minimus are extended slightly southwest from northeastern Colorado. Compared to other coeval North American faunas, the Florissant mammalian fauna is most similar to more northern faunas (including those of Montana, Nebraska, Wyoming, Saskatchewan, and NE Colorado). Geographically and faunally, the Florissant fauna appears to straddle the border between the Rocky Mountain and Great Plains Provinces defined by others, and appears consistent with weakened faunal provinciality during the Chadronian.