The hydrochemistry and the microbial diversity of a pristine aquifer system near Garzweiler, Germany, were characterized. Hydrogeochemical and isotopic data indicate a recent activity of sulfate-reducing bacteria in the Tertiary marine sands. The community structure in the aquifer was studied by fluorescence in situ hybridization (FISH). Up to 7.3 x 10(5) cells/mL were detected by DAPI-staining. Bacteria (identified by the probe EUB338) were dominant, representing 51.9% of the total cell number (DAPI). Another 25.7% of total cell were affiliated with the domain Archaea as identified by the probe ARCH915. Within the domain Bacteria, the beta-Proteobacteria were most abundant (21.0% of total cell counts). Using genus-specific probes for sulfate-reducing bacteria (SRB), 2.5% of the total cells were identified as members of the genus Desulfotomaculum. This reflects the predominant role these microorganisms have been found to play in sulfate-reducing zones of aquifers at other sites. Previously, all SRB cultured from this site were from the spore-forming genera Desulfotomaculum and Desulfosporosinus.