Desert tortoises (Gopherus agassizii) occurring east and south of the Colorado River form the "Sonoran population," a regulatory designation of the U.S. Fish and Wildlife Service, whereas tortoises west and north of the river constitute the "Mojave population." This distinction is based on significant genetic, morphometric and ecological differences. However, mitochondrial DNA, morphometric, and ecological data from the eastern bajada of the Black Mountains (about 40 km east of the Colorado River) identify the evolutionary affinities of those tortoises as Mojavean: ten of eleven Black Mountain tortoises possessed the Mojave genotype, twenty-four of thirty-seven tortoises predominantly expressed the Mojave phenotype, and all tortoises were similar to Mojave populations in macrohabitat selection. Some ecological and behavioral attributes such as home range size and hibernaculum selection did not differ among Mojave, Sonoran, and Black Mountain tortoises. Several hypotheses on how the Mojave trait became established in the Black Mountains are discussed.