To gain more insight into the competitive and syntrophic interactions between sulfate-reducing and methanogenic consortia in sulfate-limited anaerobic reactors, the microbial population in granular sludge from a full-scale upflow anaerobic sludge blanket (UASB) reactor was studied, using the 16S rRNA dot-blot hybridization method in combination with most probable number estimates. The wastewater which was treated in the UASB reactor contained mainly starch, acetate, propionate, butyrate and formate, and had a chemical oxygen demand/sulfate ratio of 9.5. Evidence was obtained that acetate was mainly degraded by Methanosaeta-like microorganisms, while propionate was the preferred substrate for sulfate reduction. The Desulfobulbus-like propionate-degrading sulfate reducers in the sludge competed with Syntrophobacter-like bacteria for the available propionate. Hydrogen and formate were probably mainly degraded via methanogenesis by members of the order Methanobacteriales. Hydrogen, formate and butyrate-degrading sulfate reducers could not be characterized with the 16S rRNA probes available to date. The same was true for syntrophic butyrate degraders. z