From extreme southern Colorado south across New Mexico to the Mexican border _T. sirtalis_ occurs in continuous or nearly continuous populations in the Rio Grande Valley, and has been recorded from many localities. It has been recorded from relatively few localities of tributary streams (Los Pinos, Abiqui, Santa Fe) all near the main valley. There is one record from the Ocate River, a headwaters tributary of the Canadian River, in the Sangre de Cristo Mountains near other localities in the Rio Grande drainage. The southwestern-most known locality of occurrence is Casas Grandes in the Mexican state of Chihuahua some 130 miles southwest of El Paso, Texas, and near the Continental Divide. The Rio Casas Grandes must have once been a tributary of the Rio Grande, but now its desert drainage basin is isolated. RE-DESCRIPTION OF A SUBSPECIES FROM NEW MEXICO Most specimens of a population of _sirtalis_ occurring in New Mexico are recognizably different from most specimens of other populations. This New Mexican population is therefore here recognized as a distinct subspecies: THAMNOPHIS SIRTALIS ORNATA Baird _Eutaenia ornata_ Baird, 1859:16. _Eutaenia sirtalis dorsalis_ Cope, 1900:1076. _Thamnophis sirtalis parietalis_ (part) Van Denburgh, 1924:222. _Type._--U. S. Nat. Mus. No. 960, obtained at El Paso, Texas, at some time in the eighteen fifties by Col. J. D. Graham. _Range._--Rio Grande and vicinity, from Conejos and Costilla counties in extreme south-central Colorado south across New Mexico to Mexican border. Records from neighboring drainage systems, Casas Grandes in Chihuahua and Artesia and Ocate River in New Mexico, probably also pertain to _ornata_.