A well-preserved ungual of a pes documents the presence of Jefferson's ground sloth (Megalonyx jeffersonii) at the end of the Wisconsinan in North Dakota. This is the 1st report of M. jeffersonii in North Dakota, and one of few records from the upper Great Plains. An accelerator mass spectrometer radiocarbon age of 11,915 ?? 40 years ago was obtained from the specimen, suggesting that the sloth resided in North Dakota during the Rancholabrean Land Mammal Age, just before extinction of the species. Palynological records from sites near the sloth occurrence and of the same age indicate that it resided in a cool, moist, spruce-dominated forest habitat in a riparian setting along the Missouri River. Its presence in that setting corroborates the notion that Jefferson's ground sloth was a browsing inhabitant of gallery forests associated with rivers. It is likely that M. jeffersonii used river valleys, such as the Missouri River valley, as migration routes. ?? 2007 American Society of Mammalogists.