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Difficulties in the subspecific identification of more than 200 specimens of Thomomys umbrinus from the State of Tlaxcala led us to examine the systematic status of the eight subspecies recognized within the Mexican Transvolcanic Belt. We gathered representative samples of those subspecies (≥ 50) during three years of fieldwork. These were analyzed together with samples from other populations not previously assigned to subspecies, as well as voucher specimens from several collections. Thirty-seven variables, obtained from 1145 specimens, were analyzed morphometrically using univariate and multivariate statistics. Within-population variation was highly influenced by sexual dimorphism, and especially by ontogenetic variation; therefore, these two factors should be considered carefully when making systematic and taxonomic decisions. Our analysis of geographic variation determined only two recognizable subspecies of Thomomys umbrinus within the Mexican Transvolcanic Belt: Thomomys umbrinus umbrinus (Richardson, 1829) and Thomomys umbrinus pullusHall and Villa, 1948.