The common garter snake (Thamnophis sirtalis) has by far the most extensive geographic range of any North American reptile, covering most of the continental United States from the Atlantic to the Pacific and from south of the Mexican boundary far north into Canada and southeastern Alaska. Of the several recognized subspecies, the eastern T. s. sirtalis has the most extensive range, but that of T. s. parietalis in the region between the Mississippi River and the Rocky Mountains is almost as large. The more western T. s. fitchi occurring from the Oregon and California coasts east through the northern Great Basin, has the third largest range, while the far western subspecies pickeringi, concinnus, infernalis and tetrataenia, and the Texan T. s. annectens all have relatively small ranges.

Since the publication of Ruthven's revision of the genus Thamnophis more than 50 years ago, little attention has been devoted to the study of this widespread and variable species, except in the Pacific Coast states (Van Denburgh, 1918; Fitch, 1941; Fox, 1951). However, Brown (1950) described the new subspecies annectens in eastern Texas, and many local studies have helped to clarify the distribution of the species in the eastern part of the continent and to define the zone of intergradation between the subspecies sirtalis and parietalis. In our study attention has been focused upon parietalis in an attempt to determine its western limits and its relationships to the subspecies that replace it farther west.