Several cultures ofThiobacillus ferrooxidans were purified by isolation of single colonies and simple physiological and biochemical properties of the purified cultures, such as colony morphology, kinetics of ferrous ion oxidation, growth on elemental sulfur and thiosulfate, oxidation of ferrous ion by growing cells in the presence of organic compounds and metal ions, were examined. These properties may be used to characterize a range ofT. ferrooxidans isolates and for numerical taxonomy.