A benzaldehyde-degrading enrichment culture was obtained from mesophilic methanogenic granular sludge from a UASB (upflow anaerobic sludge bed) reactor treating potato starch wastewater. The enrichment culture degraded benzaldehyde rapidly to benzylalcohol and benzoate. Two benzaldehyde-degrading bacterial strains were isolated from the enrichment. Both strains could grow on yeast extract in the presence or absence of benzaldehyde. Strain BOR (DSM 12 858) was able to convert benzaldehyde to benzoate and benzylalcohol. The ratio of benzoate to benzylalcohol increased with increasing concentrations of yeast extract and carbon dioxide. Benzaldehyde conversion was not growth supportive. Strain BR (DSM 12 857) reduced benzaldehyde to benzylalcohol. This conversion was coupled to growth. The bacterium used benzaldehyde as terminal electron acceptor for the oxidation of amino acids in yeast extract or molecular H2. When H2 was the electron donor the bacterium required yeast extract as carbon source.