Glucose-6-phosphate dehydrogenase (G6PD) deficiency appears to be fairly common in Mexico. We have now examined the DNA of three previously reported electrophoretically fast Mexican G6PD variants, -G6PD Distrito Federal, G6PD Tepic, and G6PD Castilla. All three of these variants, believed on the basis of biochemical characterization and population origin to be unique, have the G----A transition at nucleotide 202 and the A----G transition at nucleotide 376, mutations that we now recognize to be characteristic of G6PD A-. Two other Mexican males with G6PD deficiency were found to have the same mutation. All five have the (NlaIII/FokI/PvuII/PstI) haplotype characteristic of G6PD A -in Africa. Since the PvuII+ genotype seems to be rare in Europe, we conclude that all of these G6PD A - genes had their ancient origin in Africa, although in many of the Mexican patients with G6PD A -202A/376G the gene may have been imported more recently from Spain, where this variant, formerly known as G6PD Betica, is also prevalent.