Comparative analysis of 16S rRNA gene sequences, DNA-DNA hybridization data and phenotypic properties revealed that 'Sulfobacillus thermosulfidooxidans subsp. thermotolerans' strain K1 is not a member of the genus Sulfobacillus. Phylogenetically, strain K1 is closely related to unclassified strains of the genus Alicyclobacillus: the 16S rRNA gene sequence of strain K1 is similar to that of Alicyclobacillus sp. AGC-2 (99.6 %), Alicyclobacillus sp. 5C (98.9 %) and Alicyclobacillus sp. CLG (98.6 %) and bacterium GSM (99.1 %). The 16S rRNA gene sequence similarity values for strain K1 and species of the genus Alicyclobacillus with validly published names were in the range 92.1-94.6 %, and for S. thermosulfidooxidans VKM B-1269(T) the value was 87.7 %. Sulfobacillus disulfidooxidans SD-11(T) was also phylogenetically related to strain K1 (92.6 % sequence similarity) and thus belonged to the genus Alicyclobacillus. Chemotaxonomic data, such as the major cell-membrane lipid components of strains K1 and SD-11(T) (omega-alicyclic fatty acids) and the major isoprenoid quinone (menaquinone MK-7) of strain K1, supported the affiliation of strains K1 and SD-11(T) to the genus Alicyclobacillus. Physiological and molecular biological tests allowed genotypic and phenotypic differentiation of strains K1 and SD-11(T) from the nine Alicyclobacillus species with validly published names. The G+C content of the DNA of strain K1 was 48.7+/-0.6 mol%; that of strain SD-11(T) was 53+/-1 mol%. DNA-DNA reassociation studies showed low relatedness (22 %) between strains K1 and SD-11(T), and even lower relatedness (3-5 %) between these strains and Alicyclobacillus acidocaldarius subsp. acidocaldarius ATCC 27009(T), DSM 446(T). DNA reassociation of strains K1 and SD-11(T) with Alicyclobacillus cycloheptanicus DSM 4006(T) gave values of 15 and 21, respectively. Based on the phenotypic and phylogenetic characteristics of strains K1 and SD-11(T), Alicyclobacillus tolerans sp. nov. (type strain, K1(T)=VKM B-2304(T)=DSM 16297(T)) and Alicyclobacillus disulfidooxidans comb. nov. (type strain, SD-11(T)=ATCC 51911(T)=DSM 12064(T)) are proposed.