An α-glucosidase producing, thermophilic, facultatively anaerobic, and endospore-forming, motile, rod-shaped bacterial strain F84b^T was isolated from a high temperature well-pipeline sediment sample in Kizilcahamam, Turkey. The growth occurred at temperatures, pH and salinities ranging from 45 to 69ºC (optimum 60ºC), 7.0 to 8.5 (optimum 8.0) and 0 to 5% (w/v) (optimum 3.5%), respectively. Strain F84b^T was able to grow on a wide range of carbon sources. Starch and tyrosine utilization, amylase, catalase and oxidase activities, nitrate reduction, and gas production from nitrate were all positive. The G+C content of the genomic DNA was 49.6 mol%. The menaquinone content was MK-7. The dominant cellular fatty acids were iso-C17:0, iso-C15:0, and C16:0. In phylogenetic analysis of 16S rRNA gene sequence, strain F84b^T showed high sequence similarity to Geobacillus thermodenitrificans (99.8%) and to Geobacillus subterraneus (99.3%) with DNA hybridization values of 74.3% and 29.1%, respectively. In addition, the Rep-PCR and the intergenic 16S-23S rRNA gene fingerprinting profiles differentiated strain F84b^T from the Geobacillus species studied. The results obtained from the physiological and biochemical characters, the menaquinone contents, the borderline DNA-DNA hybridization homology, and the genomic fingerprinting patterns had allowed phenotypic, chemotaxonomic and genotypic differentiation of strain F84b^T from G. thermodenitrificans. Therefore, strain F84b^T is assigned to be a new subspecies of G. thermodenitrificans, for which the name Geobacillus thermodenitrificans subsp. calidus, subsp. nov. is proposed (The type strain F84b^T= DSM 22629^T = NCIMB 14582^T).