The nucleotide sequence of the 16S rRNA gene of the type strain Sulfobacillus thermosulfidooxidans VKM-1269 was determined. This microorganism belongs to the group of thermophilic acidophilic chemolithotrophic bacteria capable of utilizing elemental sulfur and iron ions as the sole energy sources. The phylogenetic position of the bacterium studied among Gram-positive bacteria was estimated by a comparative analysis of 16S rRNA nucleotide sequences. It was shown that Sulfobacillus forms a coherent group with species of a new genus Alicyclobacillus. The Sulfobacillus-Alicyclobacillus group is suggested to represent a separate evolutionary branch within the clostridial subdivision of Gram-positive bacteria.