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2 Abstract The objectives of this study were to analyze the population structure and genetic variability of two communities, Costa da Lagoa (CLG) and Sao Joao do Rio Vermelho (SJRV), located on Santa Catarina Island in southern Brazil. The two populations descend from Azores Archipelago im- migrants (Portuguese), with a minor contribution of sub-Saharan Africans and Amerindians. To estimate the relative contribution of the different ethnic groups to the current gene pool of the two communities, values of admixture were obtained using the weighted least-squares method based on allelic fre- quencies of the loci ABO, RHD-RHCE, GPA-GPB (MNSs), HBB, HP, TF, CP, AK, and ACP1. The origins of the studied populations can be quantified as fol- lows: for CLG, sub-Saharan Africans (A) = 17.3%, Iberian Europeans (P) = 75.0%, and Southern Amerindians (I) = 7.7%; for SJRV, A = 48.8%, P = 44.5%, and I = 6.7%. Because haplotype frequencies of the GPA-GPB loci in SJRV were unusual, possibly as a consequence of random genetic drift, the values of admixture were recalculated after exclusion of GPA-GPB, as fol- lows: A = 28.0%; P = 53.3%, and I = 18.7%. The total diversity (HT) was esti- mated as 42.29%, of which 99.6% can be attributed to the intrapopulational variability (HS). The interpopulational genetic variation (or standard distance, DST) corresponds to 0.19%, while the gene differentiation coefficient is 0.28%, indicative of low genetic difference. These results led to the conclusion that random genetic drift may have had an important effect on the Costa da Lagoa community, while presently gene flow might be the predominant evolu- tionary factor potentially capable of changing allele frequencies in SJRV. The communities of Costa da Lagoa (CLG) and Sao Joao do Rio Vermelho (SJRV) descend from immigrants of the Azores Archipelago (Portuguese), sub