Hipparionin material recovered from the middle Miocene El Camarón Formation (late early Barstovian, K-Ar dated 15. 0 ± 0.8 to 16.7 ± 0.71 Ma) of the state of Oaxaca, southeastern Mexico, is formally described. The sample consists of upper and lower cheek teeth, whose sizes and occlusal patterns are closest to those of the North American Cormohipparion quinni from the late Barstovian of the Great Plains. The material is referred to C. aff. C. quinni and constitutes one of the southernmost occurrences of the genus, extending its geographic distribution from the Northern Great Plains to southeastern Mexico during the middle Miocene. The dentally derived species reported here is contem poraneous with C. goorisi from the early Barstovian of Texas and Florida, the oldest and most plesiomorphic known species, thus lending support to the hypothesis that part of the hipparionin differentiation probably took place in southern (tropical) North America. This in turn leads to a reappraisal of where and when the radiation of some middle Miocene hypsodont horses actually occurred.