The distribution and diversity of Danish freshwater snails is discussed in the light of MacArthur and Wilson's theory of island biogeography. Species-area curves for 86 eutrophic and oligotrophic ponds and lakes are made and the variance to mean ratios of species numbers in lakes of identical sizes calculated. These data indicate that the snail fauna represents a dynamic equilibrium sustained by immigration and extinction. Different properties of small and large bodies of water are discussed with regard to probability of immigration and extinction. Oligotrophic lakes show a steeper species-area curve than eutrophic ones. This is explained by a higher extinction probability in the former but the total species pool is the same for both types of lakes.The freshwater snails are described as a group largely consisting of fugitive species. Physicochemical factors set the ultimative physiological limits to a species but within the main area of distribution biotic interrelations may be of main importance in governing the diversity of local faunas. These considerations probably have a general application to the hololimnic fauna.