Mexican Americans residing in Starr County, Texas, were grouped by their year of birth (1896–1925, 1926–1955, and 1956–1985) to determine the extent of birth cohort‐related genetic variation within this population and the genetic differences, if any, from the Mexican population residing in the Metropolitan Monterrey Area (MMA), Nuevo León, México. Twenty‐one genetic markers were analyzed which indicate that the three birth cohort groups are genetically indistinguishable. Gene diversity analysis suggests that more than 99.8% of the total gene diversity can be attributed to variation between individuals within the birth cohort populations and that the subdivision by birth cohort has only a small contribution (0.18%) to the total gene diversity. Genetic admixture analysis indicates a predominant influence from the Spanish, and that the three birth cohort groups were similar in terms of contributions of this ancestral population. The genetic structure of the Mexican American population of Starr County was also similar to the Mexican population from the State of Nuevo León, México. These findings, together with previous results, suggest that the Mexican Americans of Starr County, Texas, classified by gender, birthplace, and age, are not genetically distinguishable and are similar to the Mexican populations of the State of Nuevo León. © 1994 Wiley‐Liss, Inc.