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Most of the inhabitants of South Tyrol in the eastern Italian Alps can be considered isolated populations because of their physical separation by mountain barriers and their sociocultural heritage. We analyzed the genetic structure of South Tyrolean populations using three types of genetic markers: Y-chromosome, mitochondrial DNA (mtDNA), and autosomal Alu markers. Using random samples taken from the populations of Val Venosta, Val Pus-teria, Val Isarco, Val Badia, and Val Gardena, we calculated genetic diversity within and among the populations. Microsatellite diversity and unique event polymorphism diversity (on the Y chromosome) were substantially lower in the Ladin-speaking population of Val Badia compared to the neighboring German-speaking populations. In contrast, the genetic diversity of mtDNA haplotypes was lowest for the upper Val Venosta and Val Pusteria. These data suggest a low effective population size, or little admixture, for the gene pool of the Ladin-speaking population from Val Badia. Interestingly, this is more pronounced for Ladin males than for Ladin females. For the pattern of genetic Alu variation, both Ladin samples (Val Gardena and Val Badia) are among the samples with the lowest diversity. An admixture analysis of one German-speaking valley (Val Venosta) indicates a relatively high genetic contribution of Ladin origin. The reduced genetic diversity and a high genetic differentiation in the Rhaetoroman-and German-speaking South Tyrolean populations may constitute an important basis for future medical genetic research and gene mapping studies in South Tyrol.