Two moderately thermophilic, Gram-positive, spore-forming bacteria were isolated from different geographical locations and sources; strain GS5-97^T from a beet sugar factory in Leopoldsdorf, Lower Austria, and strain YNP10 from a geothermally heated soil, Yellowstone National Park, USA. The sequences of their 16S rRNA genes were found to be 99.8% identical, and DNA–DNA hybridization experiments revealed that strains GS5-97^T and YNP10 share 89.9 mol% similarity to each other, but only 34.3 and 39.2 mol% similarity, respectively, to Geobacillus caldoxylosilyticus DSM 12041^T, which is their closest related type strain. A polyphasic analysis showed that these two isolates were more similar to each other than to other characterized geobacilli. Their DNA G+C content was 43.2 and 42.4 mol%, respectively, and they were identical with respect to many phenotypic features (e.g. T_opt 55 °C; pH_opt 7.0). Both strains clearly displayed best growth when cultured aerobically. They differed slightly in their cellular fatty acid profiles and polar lipid pattern, and genotypically they could also be distinguished based on randomly amplified polymorphic DNA fingerprints and internal transcribed spacer analysis. Freeze-etching experiments revealed oblique surface layer (S-layer) lattices in both strains, and biochemical analyses of the purified S-layer proteins indicated the occurrence of glycosylation. Based on the properties of these organisms relative to those currently documented for the genus Geobacillus and for the various sister genera in the Bacillus radiation, a novel species is proposed, Geobacillus tepidamans sp. nov., with GS5-97^T (=ATCC BAA-942^T=DSM 16325^T) as the type strain. Strain YNP10 has been deposited in the American Type Culture Collection as ATCC BAA-943.