Two anaerobic, benzaldehyde-converting bacteria were isolated from an anaerobic upflow anaerobic sludge bed (UASB)-reactor treating potato starch waste water. Strain BOR-Y(T) converted benzaldehyde to benzoate and benzylalcohol in approximately equimolar concentrations. Benzaldehyde conversion did not support growth. Strain BOR-Y(T) was Gram-positive and rod-shaped, and its cells were slightly thickened in the middle. The strain was a mesophilic spore-former that grew between 15 and 40 degrees C, with optimum growth at 30-37 degrees C. The optimum pH for growth was pH 7.0. Strain BOR-Y(T) grew on a wide range of carbohydrates and some other carbon sources including yeast extract, cysteine and serine. The G+C content of its DNA was 42 mol%. According to physiological characteristics and 16S rRNA gene sequence analysis, confirmed by DNA-DNA hybridization with its phylogenetic neighbours, strain BOR-Y(T) belongs to a novel genus of cluster XII of the clostridia, namely Soehngenia; the name Soehngenia saccharolytica is proposed for the type species (type strain BOR-Y(T)=DSM 12858(T)=ATCC BAA-502(T)). Strain BR-10(T) reduced benzaldehyde to benzylalcohol. This conversion was coupled to growth. In a medium containing yeast extract, the presence of benzaldehyde resulted in the accumulation of more than twofold more cells. Strain BR-10(T) was a Gram-positive organism that was characterized by oval- or rod-shaped cells with oval ends, which occurred singly, in pairs or sometimes in chains. The strain was moderately thermophilic and grew between 20 and 60 degrees C, with optimum growth at 45 degrees C. The optimum pH for growth was between pH 7.0 and 7.5. Strain BR-10(T) grew on a wide range of carbon sources including carbohydrates, yeast extract, casein and some amino acids. The G+C content of its DNA was 32 mol%. As determined by 16S rRNA gene sequence analysis, strain BR-10(T) represents a novel species of cluster XIVa of the clostridia; the name Clostridium amygdalinum is proposed for this novel species (type strain BR-10(T)=DSM 12857(T)=ATCC BAA-501(T)).