This species is known only from a limited area in extreme southwestern United States, and extreme northwestern Mexico. The range includes southeastern California, extreme northeastern Baja California (Grismer 2002), northwestern Sonora, and southwestern Arizona (Funk 1981, Stebbins 2003). In California, the species ranges southward from the Coachella Valley, including both sides of the Salton Sea and Imperial Valley, and westward into the Borrego Valley, Ocotillo Wells area, West Mesa, and Yuha Desert (Yuha Basin), and, on the east side of the Imperial Valley, to the vicinity of the Dos Palmas Bureau of Land Management (BLM) Area of Critical Environmental Concern (ACEC), but predominantly it occurs in East Mesa and in areas adjoining the Algodones Dunes (i.e., Imperial Sand Dunes, Glamis Sand Dunes) on the east side of the Imperial Valley. In southwestern Arizona, it occurs south of the Gila River and west of the Gila and Tinajas Altas Mountains in Yuma County (Rorabaugh et al. 1987). The range extends into Mexico from the international border in the Yuha Desert in California, south to Laguna Salada in Baja California, and from the international border in the Yuma Desert in Arizona, south and east through the Pinacate Region to the sandy plains around Puerto Penasco and Bahia de San Jorge, Sonora (Johnson and Spicer 1985, Gonzales-Romero and Alvarez-Cardenas 1989; these were cited, without full literature citations, by USFWS 2003). The distribution of the Flat-tailed Horned Lizard is not contiguous across its range; it is fragmented by large-scale agricultural and urban development, primarily in the Imperial Valley and the Coachella Valley. In addition, the Salton Sea, Colorado River, East Highline Canal, New Coachella Canal, and All American Canal are barriers to movement. Due to this habitat fragmentation and existing geographic barriers, the United States distribution appears to be currently divided on a broad scale into at least four geographically discrete subpopulations: three in California and one in Arizona. The three subpopulations in California are located in the Coachella Valley, the west side of the Salton Sea/Imperial Valley, and the east side of the Imperial Valley [from USFWS 2003]. The Flat-tailed Horned Lizard has been recorded at elevations as high as 520 m (1,706 feet) above sea level, but is more commonly found below 250 m (820 feet) in areas with flat-to-modest slopes (Turner et al. 1980). See Turner and Medica (1982) for the results of surveys done in southeastern California in 1978-1980; the area north of Highway 78 in the vicinity of Ocotillo Wells and Benson Dry Lake (= Ocotillo Dry Lake), eastern San Diego County, was identified as a particularly favorable area; other areas of relatively high abundance were in Imperial County--southern East Mesa, southeastern Yuha Desert, and the Superstition Mountain area. In Arizona, an area southeast of Yuma registered a relatively high abundance index (Rorabaugh et al. 1987). Its area of occupancy is very small within the extent of occurrence because of its dependence on wind-blown sand.