An integrated study, combining biogeochemical, stable isotope, micro-sensor, sedimentological, phase-analytical, and molecular ecological methods, was carried out in April 1998 in a temperate intertidal mudflat (Site Dangast; German Wadden Sea of the southern North Sea). The biogeochemical zonation was investigated in relation to the vertical abundance of total and sulfate-reducing bacteria, crustaceans, nematodes, flagellates, and ciliates. Total organic carbon (TOC) contents of the sediments ranged between 1.0 and 3.3% dry weight and were related to the abundance of clay minerals, indicating sorption processes on mineral surfaces to control organic matter burial. The sediments above 9 cm below sea floor contained an excess of TOC compared to the relationship between TOC and pyrite sulfur proposed for normal marine sediments. The downcore variation of the carbon isotopic composition of organic matter reflected the preferential microbial degradation of labile (marine) organic matter relative to a more resistent (terrestrial) organic matter fraction. The oxygen penetration depth was 4.6 mm in the light and 1.2 mm in the dark, and coincided with the maximum abundance of ciliates, crustaceans and heterotrophic flagellates. Although sub-oxic conditions were indicated by the presence of dissolved Fe(II) and Mn(II) to about 15 cm depth, bacterial sulfate reduction rates between 14 and 225 nmol cm−3 d−1 were measured using radio-tracers with a first maximum at around 2 cm depth. Up to 80% of the total cells as detected by DAPI-staining hybridized with a rRNA-targeted oligonucleotide probe specific for the domain bacteria (EUB338). Sulfate-reducing bacteria as detected by probe SRB385 showed high abundance (up to 7% of total cells) in the upper 5 cm of the sediment. Total and cell numbers of sulfate reducers were highest at about 2 cm and decreased with depth. Cellular sulfate reduction rates were estimated from the SRB counts by fluorescence in situ hybridization and the measured sulfate reduction rates and ranged between 0.06 and 0.55 fmol SO42− cell−1 day−1 which is at the lower end determined for pure cultures. From a comparison of cellular SRR and stable sulfur isotope (/) fractionation between coexisting dissolved pore water sulfate and sedimentary reduced sulfur species with laboratory studies a significant contribution of bacterial disproportionation reactions within the oxidative part of the sedimentary sulfur cycle is indicated.