Most large collections of garter snakes contain misidentified specimens. The diagnostic differences in color and pattern are often obscured, especially if the specimens are poorly preserved. Many specimens deviate from the scalation typical of the form they represent, and key out to other species. Isolated records should therefore be accepted with caution. A case in point is Colorado University Museum No. 46, from Buford, Rio Blanco County, Colorado, originally identified by Cockerell (1910:131) as _Thamnophis sirtalis parietalis_. This specimen, and another, now lost, from Meeker in the same county seemingly served as the basis for mapping the range of _sirtalis_ across the western half of Colorado, for there seem to be no other records from this part of the state. However, a re-examination of the specimen from Buford shows it to be an atypical individual of another species, _T. elegans vagrans_. A specimen of _T. radix haydeni_ (Col. U. Mus. No. 3165) was the basis for Maslin's (1959:53) record of _parietalis_ in Baca County on the north fork of the Cimarron River in southeastern Colorado. Brown (1950:203) has mentioned the difficulty of defining the range of _sirtalis_ in the southern Great Plains because of misidentifications of the similar _T. radix_. The range of the common garter snake has never been adequately mapped in the Rocky Mountain and Great Basin states. Recent general works (Smith, 1956:291; Wright and Wright 1957:834; Stebbins 1954:505; Conant 1958:328) which have shown maps of the over-all range of _sirtalis_, differ sharply as to the extent of its distribution in Texas, New Mexico and Arizona, but all show its distribution as continuous over the more northern Great Basin and Rocky Mountain states. However, specimens and specific locality records from this extensive area seem to be scarce and some are based on early collections of doubtful provenance. Throughout this region the low rainfall, fluctuating and uncertain water supply, and general lack of mesic vegetation along many of the streams render the habitat rather hostile to garter snakes in general. _Thamnophis elegans vagrans_, highly adapted to conditions in this region and generally distributed over it, doubtless offers intensive competition to the species _sirtalis_ wherever they overlap and perhaps constitutes a limiting factor for _sirtalis_ in some drainage basins.