The genetical effects of the subdivision of a population into partially isolated subgroups are considered in two part.icular cases. In the first a probability model is studied in which the subpopulations are of finite size with migration between them. In the absence of selection the asymptotic rate of progress to homozygosity is shown to be very little affected by the subdivision. In the second case a deter. ministic model is studied in which there are two subpopulations in which selective forces are equal and opposite. A stable dimorphism is then shown to exist if there is any small amount of intermigration.