In Finland there is a substantial but geographically limited Swedish-speaking minority (in 1980 6.3% of the total population) which originates mainly from Swedish immigrants during the years 1100-1300 AD. The admixture of this population with the neighbouring Finns was studied using more than 20 blood marker loci. The reference populations, Swedes and Finns, in spite of being part of the genetically rather uniform European populations, differ from each other genetically. These quantitative and also qualitative differences in gene frequencies are mostly due to the Finnish population possessing a number of genetic markers absent or rare in the rest of Europe. The results based on a sample of 620 individuals from the Swedish-speaking population in Finland showed a rather high degree of Finnish admixture, which was estimated to about 60%. This admixture most probably occurred at an early stage since it has reached such a high and geographically homogeneous degree.