A mesophilic, syntrophic acetate-oxidizing bacterium, designated strain Sp3(T), was isolated from sludge from a mesophilic methanogenic digestor operating at a high ammonium concentration (6.4 g L(-1) NH(4)(+)-N). The strain showed acetate-oxidizing ability in cocultivation with a hydrogen-consuming methanogen. Comparative 16S rRNA gene sequence analysis confirmed that strain Sp3(T) belonged to the Firmicutes-Clostridia class. The most closely related species was Thermacetogenium phaeum (16S rRNA gene sequence identity 92%). Strain Sp3(T) used ethanol, betaine and lactate as carbon and electron sources and showed growth between 25 and 40 degrees C and pH 6.0 and 8.0. Based on the phylogenetic position and the physiological characteristics of strain Sp3(T), this new syntrophic, acetate-oxidizing bacterium is proposed as the new genus and species Syntrophaceticus schinkii, with Sp3(T) (=JCM 16669(T)) as the type strain. An isolate (strain Esp=JCM 16670) with high 16S rRNA gene sequence identity (99%) to syntrophic acetate-oxidizing Clostridium ultunense was also retrieved from the methanogenic digestor.