We have characterized 68 unrelated Basque individuals from Vizcaya, Spain, for 13 tetrameric short tandem repeat (STR) loci: CSF1PO, D3S1358, D5S818, D7S820, D8S1179, D13S317, D16S539, D18S51, D21S11, FGA, TH01, TPOX, and VWA. Interpopulational analyses were also performed for 21 European and North African population data sets for nine of the STRs typed in the Basque sample. Heterozygosity values for the Vizcayan Basques were found to be high, ranging from 0.662 to 0.882, and none of the STR loci significantly deviated from Hardy-Weinberg equilibrium. Based on the comparative population data set, the average G_ST score is 0.7%, indicating a low degree of genetic differentiation. However, neighbor-joining trees and multidimensional-scaling plots of D_A genetic distances indicate that the Vizcayan Basques are an outlier relative to both neighboring Iberians and North African populations.