A novel thermophilic spore-forming strain, Ga(T), was isolated from the Garga hot spring located in the northern part of the Transbaikal region (Russia). Strain Ga(T) was found to be an aerobic, Gram-positive, rod-shaped, thermophilic (optimum growth temperature is 60-65 degrees C), chemo-organotrophic bacterium that grows on various sugars, carboxylic acids and hydrocarbons. The G+C content of its DNA is 52.9 mol%. The 16S rRNA gene sequence similarity data show that strain Ga(T) is closely related to members of the genus Geobacillus. Relevant chemotaxonomic data (in particular, the major fatty acid profile of strain Ga(T), which includes iso-C15 : 0, iso-C16 : 0 and iso-C17 : 0 acids) support the assignment of this strain to the genus Geobacillus. The physiological, biochemical and DNA-DNA hybridization studies of strain Ga(T) showed that it differs both genotypically and phenotypically from the recognized Geobacillus species. Based on these data, strain Ga(T) belongs to a novel species, Geobacillus gargensis sp. nov. (type strain, Ga(T)=VKM B-2300(T)=DSM 15378(T)). The analysis of the phenotypic characteristics (additional to those given in the original description) of the type strain of Bacillus vulcani (DSM 13174(T)) showed that they are very similar to the major phenotypic characteristics of the genus Geobacillus. The low DNA-DNA reassociation values of strain DSM 13174(T) with various species of this genus (from 38 to 54 %) clearly demonstrate a sufficient genomic distinction of this strain and its taxonomic status as a species. The physiological characteristics, phylogenetic position and DNA-DNA reassociation values of B. vulcani allow this species to be reclassified as Geobacillus vulcani comb. nov. The main properties that differentiate G. vulcani from the other species of the genus are its ability to produce acids from glycerol, lactose and ribose.