One thousand individuals from the southern population of Porto Alegre and 760 from the northeastern city of Natal were studied in relation to 12 and 8 genetic systems, respectively. The data thus gathered were used in different ways to estimate quantitatively the ethnic composition of individuals from these communities. More than half of the genes present in individuals classified as Black in Porto Alegre may be of White origin, while the Whites from this city have 8% of African alleles. The estimated degree of admixture in persons identified as White or Mixed in Natal is not much different among themselves. The ancestry of the total sample can be characterized as 58% White, 25% Black, and 17% Indian.