A small municipality of about 2,000 inhabitants on a large plain (that of the river Po, which flows across the whole of Northern Italy) was chosen as a model to study the level of genetic isolation of a population which is not delimited by clear geographical barriers. Wright’s treatment of isolation by distance was considered to be applicable to this case. Estimates of the non-random component of inbreeding and of the immigration rate in the past showed that, despite the deeply rural environment, the population turnover in the area was extremely rapid. Although the parameter estimates were computed on the basis of little direct empirical evidence, it was concluded that the effective population size was at least one order of magnitude larger than might appear when considering the total population size of the municipalit