In Chile, the Hispanic dual surname system is used. To describe the isonymic structure of this country, the distribution of 16,277,255 surnames of 8,178,209 persons was studied in the 15 regions, the 54 provinces, and the 346 communes of the nation. The number of different surnames found was 72,667. Effective surname number (Fisher's α) for the entire country was 309.0, the average for regions was 240.8 ± 17.6, for provinces 209.2 ± 8.9, and for communes 178.7 ± 4.7. These values display a variation of inbreeding between administrative levels in the Chilean population, which can be attributed to the ‘Prefecture effect’ of Nei and Imaizumi. Matrices of isonymic distances between units within administrative levels were tested for correlation with geographic distance. The correlations were highest for provinces (r = 0.630 ± 0.019 for Euclidean distance) and lowest for communes (r = 0.366 ± 0.009 for Lasker's). The geographical distribution of the first three-dimensions of the Euclidean distance matrix suggests that population diffusion may have taken place from the north of the country toward the center and south. The prevalence of European plus European-Amerindian (95.4%) over Amerindian ethnicity (4.6%, CIA World Factbook) is compatible with diffusion of Caucasian groups over a low-density area populated by indigenous groups. The significant excess of maternal over paternal indigenous surnames indicates some asymmetric mating between nonAmerindian and Amerindian Chileans. The available studies of Y-markers and mt-markers are in agreement with this asymmetry. In the present work, we investigate the Chilean population with the aim of detecting its structure through the study of isonymy (Crow and Mange,1965) in the three administrative levels of the nation, namely 15 regions, 54 provinces, and 346 communes. Am J Phys Anthropol, 2012. © 2012 Wiley Periodicals, Inc.