Geographic variation in lepidosis and color pattern was studied in the nominal species Trimorphodon collaris, fasciolata, forbesi, latifascia, tan, and iipsilon. Specimens were grouped into seven geographically separated samples which were then compared. Ranges of most characteristics overlap broadly; others vary independently and discordantly. Ventral-subcaudal scales decrease from north to south. Snakes from the Balsas Tepalcatepec Basin have fewer dorsal bands than do snakes from other areas. They also have a relatively uniform head pattern. Specimens from other areas have a complex head pattern consisting of prefrontal bars, interocular bars, and discrete parietal marks in various combinations. Head pattern and body band analyses indicate limited genetic exchange between the Balsas Tepalcatepec Basin samples and those from other areas, apparently the result of secondary contact in southern Jalisco and adjacent Michoacan. The Transverse Volcanic Range and the Sierra Madre del Sur apparently are effective in restricting genetic exchange. The above nominal species are assigned to the synonymy of Trimorphodon tau. Trimorphodon tau tau is maintained for the foothills and plateau populations and T. tau latifascia (new combination) is assigned to the Balsas-Tepalcatepec Basin populations. Trimorphodon tau is redescribed and its distribution delimited. Character variation in a brood from Puebla approaches the variation typical of the entire Puebla sample.