The Troublesome Formation of northcentral Colorado ranges in age from the middle Arikareean North American Land Mammal Age (NALMA: latest Oligocene) to the early Clarendonian NALMA (early–late Miocene), a range of 14.5 million years. The rodent fauna from this formation varies greatly through the section, which is dominated by entoptychine geomyids in the Arikareean, mylagaulids in the Hemingfordian, and heteormyids in the Barstovian. The Clarendonian is the least-well represented horizon from which only four species are recognized. Of the nearly forty species of rodents identified, there is only one new genus, Argaleogaulus a primitive promylagauline mylagaulid from the Arikareean. There are 12 new species recognized: the mylagaulid Argaleogaulus primoticus; a sciurid, Protospermophilus parvus; six heteromyids, Harrymys cyanothos, Harrymys taussigi, Schizodontomys bareia, Balantiomys coloradensis, Oregonomys perilaccos, and Cupidinimus robinsoni; and four entoptychine geomyids, Pleurolicus compressus, Pleurolicus mensae, Gregorymys montanus, and Entoptychus rensbergeri. The genera from the Troublesome Formation are similar to those of equivalent faunas throughout North America. The presence of the unique species of otherwise common North American genera suggests some minor degree of isolation of the basin during the time of formation.