Fecundity, survival, immigration and emigration in a recently established population of Barnacle Goose on Gotland, Sweden, are analysed using data from 1971 to 1985. During this period numbers increased from one to 450 breeding pairs. A simulation of population growth using a deterministic population growth model showed that immigration must have occurred during this period, probably mainly during the late 1970s and early 1980s. This net influx together with a high and stable reproductive success and a high annual survival rate resulted in exponential increase in numbers. Several fecundity parameters were higher in the Gotland population than in the Arctic populations of Barnacle Goose. This successful establishment in a temperate area more than 1,300 km south of and 2,000 km away form the species' usual breeding range, questions many explanations of Arctic distributions in geese, for example the hypothesis that a too low protein content in the food plants prevents breeding at lower latitudes.