Abstract Although the isotope content of precipitation in the semi-arid regions of southern Africa is extremely variable, groundwater samples from the same district are found to have a remarkably constant isotopic composition. The oxygen-18 content of the underground water, in general, varies by about 0.5‰ in a given area. The differences that occur between different regions are sufficiently large to allow the groundwater of an area to be characterized by means of its oxygen-18 content. In order to localize the infiltration area of an aquifer, radiocarbon dating of the water is used. It appears that the groundwater contains, in general, less of the heavy isotopes than does the precipitation in the recharge area. This indicates that infiltration only takes place during periods of heavy rainfall. Examples are given where the isotope content of the groundwater is used to distinguish between different aquifers in the same region.