We report on Aspidoscelis sexlineata (six-lined racerunner) from the four known sites of occurrence in Mexico, one being a southward range extension of 74.3 km for the species. These sites are situated in the extreme eastern part of the municipality of Matamoros, Tamaulipas, and are nearest to records for the species in the United States for Brazos Island-South Padre Island, Cameron County, Texas. In both regions, A. sexlineata occurs a short distance from the seashore in habitats formed and influenced by perturbations of hurricanes and the maritime climate of the Gulf of Mexico (e.g., sand dunes devoid of congeners with openings in halophytic vegetation). Samples of A. sexlineata from Tamaulipas are statistically similar to those from South Padre Island in two meristic characters, one ratio, and snout–vent length, but significantly different in one meristic character and one ratio involving color pattern. Because similarities among specimens from the plains of southern Texas, South Padre Island, and Tamaulipas are strongly indicated, it is reasonable to allocate the three groups to A. sexlineata stephensae, Texas yellow-headed racerunner.