The bacterial community of the mineral spring Wettinquelle in the Vogtland/NW Bohemian region (German-Czech border) was characterized by sequence analysis of amplified small subunit ribosomal RNA genes. The acidulous spring water consists mostly of old groundwater from deep aquifers, which is mixed with 15--20% young water from upper groundwater horizons. The spring water contains high concentrations of iron, Ca2+ and SO4 2- ions. A remarkable attribute is the high radon activity of 27 kBq L-1 water. Free escaping spring gas consists mainly of CO2 originating from the mantle, N2 (1.2%) and traces of other gases, like methane and helium. Close relatives of Gallionella ferruginea, a micro-aerobic oxidizer of ferrous iron, contributed most to the clone library. Clones with sequences related to Thiobacillus aquaesulis, members of the Sulfuricurvum-cluster and members of several branches of the OP11 group were present in significantly lower numbers but still with some microdiversity. These bacterial groups, which contributed strongly to the clone library and have known physiology, obviously depend on the oxygen in the younger water and reduced compounds from the below.