Obligately anaerobic, Gram-stain-positive, spore-forming bacteria indistinguishable by pulsed-field gel electrophoresis were isolated from non-dairy protein shakes in bloated bottles. One of the isolates, strain IEH 97212T, was selected for further study. The strain was closely related to Clostridium sporogenes and Clostridium botulinum Group 1 based on 16S rRNA gene sequence similarities. Phylogenetic analysis also showed that strain IEH 97212T and strain PE (=DSM 18688), a bacterium isolated from solfataric mud, had identical 16S rRNA gene sequences. Strains IEH 97 212T and DSM 18 688 were relatively more thermophilic (temperature range for growth: 30-55 °C) and less halotolerant [growth range: 0-2.5 % (w/v) NaCl] than C. sporogenes and C. botulinum. They were negative for catalase, oxidase, urease and l-pyrrolidonyl-arylamidase and did not produce indole. The strains produced acid from d-glucose, maltose and trehalose, and hydrolysed gelatin, but did not hydrolyse aesculin. The end-products of growth included acetic acid, propionic acid, butyric acid, isobutyric acid, valeric acid, isovaleric acid, isocaproic acid, phenylpropionic acid, 2-piperidinone, 2-pyrrolidinone and gas(es). The predominant fatty acids were C14 : 0, C16 : 0 and C18 : 1ω9c. The genomic DNA G+C content of strains IEH 97212T and DSM 18688 was 26.9 and 26.7 mol%, respectively. According to the digital DNA-DNA hybridization data, the relatedness of these strains was 98.4 %, while they showed only 35.7-36.0 % relatedness to C. sporogenes. Based on the results of this polyphasic study, these strains represent a novel species, for which the name Clostridium tepidum sp. nov. is proposed, with the type strain IEH 97212T (=NRRL B-65463T=DSM 104389T).