Convincing records of _sirtalis_ are lacking from all of Colorado--except for those in the drainage basins of the South Platte, and the Rio Grande east of the Continental Divide--from the eastern half of Utah (east of the Wasatch Range), from New Mexico except for the Rio Grande drainage (with one record each for the Canadian and Pecos river drainages), from southwestern Wyoming (at least that part in the Colorado River drainage basin), from the western half of Oklahoma, and from Texas, except the eastern and extreme western and northern parts. The species occurs in Nevada only near that state's western and northern boundaries. The range is therefore much different than it has been depicted heretofore, with the populations living east of the Continental Divide widely separated from those to the west for the entire length of the Rocky Mountains south of the Yellowstone National Park region. The populations of northern Utah, southern Idaho, and Nevada, which have been considered _parietalis_ are thus far removed from the main population of that subspecies to the east and are isolated from them by the barrier of the Continental Divide and arid regions farther west. Although some of the records published for _Thamnophis sirtalis_ are erroneous, being based on misidentifications of other species, various outlying records, including those in western Kansas, the Panhandle of Texas, and southeastern New Mexico probably represent localized relict populations that have survived from a time when the species was more generally distributed in this region. The population of _T. sirtalis_ in the Rio Grande drainage of New Mexico is geographically isolated and remote from other populations of the species. Except for a few isolated and highly localized populations the species is absent from the Republican, Smoky Hill, Arkansas, Cimarron, Canadian, Red, Brazos, Colorado and Pecos rivers and their tributaries west of the one hundredth meridian in the arid High Plains. Streams in this region of High Plains are in most instances unsuitable habitats because they are in eroded channels, have a variable and uncertain water supply, and have poorly developed riparian communities. The marsh and wet meadow habitat preferred by _sirtalis_ in most parts of its range is almost absent. _T. radix_ and _T. marcianus_, well adapted to conditions in this region, perhaps provide competition that is limiting to _T. sirtalis_. However, several well-isolated populations of _sirtalis_ have survived as relicts in the southern Great Plains, presumably from a time several thousand years ago when mesic conditions were more prevalent, perhaps in an early postglacial stage. * * * * *