AbstractThis study is one portion of a research program on: (1) the genetic effects of transplantation of Mexican Indian populations into diverse environments; (2) the effect of Spanish colonization on the genetics of Mexico. In 1591, 91 Tlaxcaltecan families were transplanted into northern Mexico to aid in the pacification of the Chichimec Indians. The descendants of these migrants are the residents of the City of Saltillo in the state of Coahuila. Since Saltillo had undergone considerable growth due to industrialization, two adjoining barrios–Chamizal and La Minita–were chosen for this study. Chamizal is an old established barrio where the descendants of the Tlaxcaltecan colonialists resided, while La Minita is a small shantytown made up of squatters from the surrounding areas.Blood specimens were collected by venipuncture from 302 participants and analyzed for 16 blood groups, 13 serum protein and red blood cell enzyme systems. The gene frequency distribution suggests that the Saltillo populations are a triracial hybrid made up of Indian, African and Spanish parental con‐tributions. Admixture estimates and genetic distance measures are both in agreement with the ethnohistory of the transplanted Tlaxcaltecan community.