The community structure of sulfate-reducing bacteria (SRB) in an intertidal mud flat of the German Wadden Sea (Site Dangast, Jade Bay) was studied and related to sedimentary biogeochemical gradients and processes. Below the penetration depths of oxygen (similar to3 mm) and nitrate (similar to4 mm), the presence of dissolved iron and manganese and the absence of dissolved sulfide indicated suboxic conditions within the top 10 cm of the sediment. Moderate to high bacterial sulfate reduction rates were measured with radiotracers throughout the sediment, and dissimilatory sulfate reduction was also demonstrated by the presence of acid-volatile sulfides (AVS, essentially iron monosulfide). Stable sulfur isotope discrimination between dissolved sulfate and AVS was dominated by sulfate reduction. The diversity of SRB was studied using denaturant gradient gel electrophoresis of 16S rDNA, phospholipid fatty acid analysis and counting viable cells with the most probable number technique. Phylogenetic groups of SRB identified with these techniques were almost evenly distributed throughout the top 20 cm of the sediment. Application of fluorescence in situ hybridization, however, demonstrated a maximum of active members of the Desulfovibrio and Desulfosarcina-Desulfococcus- Desulfofrigus groups between 2 and 3 cm depth. These 2 groups encompass acetate and lactate utilizing SRB. The coincidence of this SRB maximum with a local maximum of sulfate reduction rates and the depletion of acetate and lactate reflects the microbiological processes related to sulfate reduction.