The karyotypic status of the mesquite lizard, Sceloporus grammicas, was investigated at two localities at Saniuario Mapethé, Hidalgo, México. A total of 23 different karyotypes was recorded from the 36 individuals examined. Individual lizards were heterozygous at up to four chromosomes. Karyotypic differences were attributed to Robertsonian fission/fusions and pericentric inversions involving the macrochromosomes. The polymorphisms generally conformed to Hardy-Weinberg equilibrium suggesting a lack of underdominance for most rearrangements. The extreme chromosomal polytypy observed at Santuario Mapethé is hypothesized to result from parapatric hybridization between two chromosome races.