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The knowledge of a population structure may constitute a pow- erful tool for mapping genes underlying susceptibility to Mendelian and complex diseases. To obtain a better understanding of the population struc- ture of São Miguel Island (Azorean Archipelago, Portugal), we carried out a surname survey using the surnames listed in the most recent telephone book (2001). We identified 1315 different surnames in a total of 27,621 sub- scribers. The frequency of the different surnames was used to calculate the following parameters: isonymy (I), random component of inbreeding (FST), genetic diversity according to Fisher (a), migration rate according to Karlin- McGregor (n) and Nei’s genetic distance. Eleven localities were selected, ac- cording to population size and geographic distribution, for analysis using the above parameters. Our results show that 51% of Salga’s population and 52% of Sete Cidades’s population are represented by six and eight surnames, re- spectively. These figures demonstrate the effective isolation of these two small places, which are located at opposite ends of São Miguel Island. Salga, Achada, and Sete Cidades present the lowest values of Fisher’s a, indicating less genetic diversity. In contrast, the capital, Ponta Delgada, presents the highest value of a (78.13), indicating more genetic diversity. Our data indi- cate that the clustering of the localities corresponds to the geographic fea- tures of the island, where localities close together tend to share similar surnames. In conclusion, the population of São Miguel is relatively homoge- neous and may constitute an ideal model for genetic mapping studies.