Abstract A strain (strain TsuHP1) of a 5-hydroxypentanoate-degrading anaerobic bacterium was isolated in pure culture from an anaerobic digestor. It was a mesophilic spore-forming gram-positive strictly anaerobic lemon-shaped bacterium. The guanine plus cytosine content of DNA was 47 mol%. The strain did not reduce nitrate, sulfate, and other sulfur compounds. It was identified as a member of the genus Clostridium. The strain grew not only on 5-hydroxypentanoate, but also on 4-aminobutanoate, 5-aminopentanoate, d-glucose, and dl-3-hydroxybutanoate. 5-Hydroxypentanoate was fermented to acetate, propionate, and valerate. d-Glucose was fermented to acetate, formate, and ethanol.