Lampropeltis alterna, L. blairi, L. thayeri, and L. greeri are reduced to subspecific status under L. mexicana on the basis of clinal variation in scutellation and coloration. Intergradation between alterna and blairi, mexicana and thayeri, and the basic morphological similarities among all considered forms furnish additional evidence of this conspecific relationship. L. mexicana differs from all other kingsnakes in having an alternating sequence of black, red, black, white, gray, white markings on the body. It is further distinguished by its wide head distinct from neck, mottled gray snout, dark postocular mark, dorsal head pattern, large eye, enlarged nuchal blotch, large number of posterior dorsal scale rows, and relatively long tail. New data on the type and only known specimen of L. leonis provide no new evidence of its relationships. This species is judged to be closest to the mexicana complex. L. mexicana occupies xeric to subhumid habitats on the Mexican Plateau and adjacent mountains from Guanajuato, Mexico, north to southwestern Texas. Its elevational range is approximately 1,300 to 7,500 feet. It is chiefly nocturnal, but feeds to a great extent on diurnal lizards.