Patterns of bacular morphology in the Eutamias quadrivittatus species complex were analyzed to determine the identity of the Eutamias population inhabiting the Organ Mountains of southcentral New Mexico. The bacula of 124 animals from all geographically adjacent taxa in this complex ( E. q. quadrivittatus , E. einereicollis cinereus , E. c. cinereicollis , E. canipes sacramentoensis , E. c. canipes , and E. bulled durangae ) were examined. Bacular data demonstrate the close phyletic relation of the Organ Mountain population with E. q. quadrivittatus , rather than with E. cinereicollis cinereus to which it is currently assigned. This conclusion is supported by karyotypic evidence. However, aspects of morphology and ecology indicate that the Organ Mountain population represents a new subspecies. The divergence of this form from E. q. quadrivittatus probably occurred in Recent time as a consequence of isolation, small population size, and a unique environment.