Summary1. Data from 14 confined populations living under natural conditions (mainly on small islands) and from a few laboratory populations have been used.2. Several differences between a typical, confined rodent population (i.e. living under natural conditions in a well isolated, homogeneous and relatively small area) and an open one have been determined.3. Island populations are characterized by:(a) Attainment and maintenance of high densities not observed at all, or recorded only periodically, in open populations of comparable species.(b) Stability of population numbers, expressed either by very slight fluctuations or by a very regular cycle with only small differences between peaks of consecutive years.(c) Lack of emigration resulting from any or all of the barriers surrounding the confined population, from homogeneity of the habitat enclosed by these barriers, or from a lower tendency to dispersal of individuals forming such populations.(d) Decreased reproduction.(e) Low losses of independent individuals.(f) Mortality of sucklings changing with the population density.(g) An age structure that is probably more differentiated and more regularly changing in an annual and long‐term cycle.(h) Different spatial organization based on smaller home ranges, that are differently arranged in relation to one another from those in open populations; the arrangement of home ranges allows the population to squeeze in a greater number of individuals with possibly the lowest number of interactions between individuals.4. In confined populations there must be different mechanisms for the regulation of numbers from those in open populations: (I) in the latter the regulation occurs by the outflow of the surplus of independent individuals, i.e. through the dispersal and high mortality of migrants; (2) in confined populations the inflow of independent individuals is regulated by controlled reproduction and early mortality of offspring. Both mechanisms are induced by social pressure but in the second case (regulation of inflow) a much stronger social organization is indispensable Only those species that are capable of the formation of such an organization can survive as confined populations.5. Knowledge of phenomena occurring in confined populations is useful for predicting the fate of populations of different species which become isolated as a result of human activity in transforming and subdividing the natural environmen