Although migrations have been thoroughly studied, still little is known about colonization processes. This study analyses the demographic and genetic structures of a colonization process. All the Welsh migrants to the Chubut (Argentina) from year 1865 until year 1925 were identified and their contribution to the gene pool of the colony estimated from genealogies. Three waves of immigration which arrived in 1865, 1874 and 1886 were the main population component, besides a high fertility. In 1868 the population was principally composed of reproductive adults. In 1876, young children and reproductive adults dominated. In 1895 it was a typical young population. In 1920 an ageing trend was already perceptible. The origins of the settlers were so varied that the colony may be considered a sampling at random in the Welsh gene pool. 40% of the founders did not contribute genes. The settling of the Welsh in the Chubut presents two characteristics that may be common to any colonization process: (1) the evolution of the demographic structure from a predominantly young adult male composition to a balance in both sexes and ages. (2) The ‘genetic cost’ resulting from the loss of the genes of some of the founders.Asociación de Antropología Biológica de la República Argentina (AABRA
