The species of the genus 'Thiobacillus' fall into the alpha-, beta- and gamma-subclasses of the Proteobacteria, the type species Thiobacillus thioparus being located in the beta-subclass. 'Thiobacillus' species exhibit almost as much diversity in DNA composition and physiology as is found collectively in all other proteobacterial groups. On the basis of physiological characters and 16S rRNA gene sequence comparisons, eight of the existing Thiobacillus species are proposed for reassignment to three newly designated genera within the gamma-subclass of the Proteobacteria, namely Acidithiobacillus, Halothiobacillus and Thermithiobacillus.