A SUBSTANCE giving a typical blue colour with the phospho-tungstate–phospho-molybdate reagent of Folin and Ciocalteu1 was found to accumulate in cultures of Thiobacillus thioparus and T. thiocyanoxidans growing in a medium containing thiosulphate. The component in question was not associated with the bacterial cells since it remained in the supernatant fluid of cultures centrifuged at high speeds. The possibility suggested itself that the material involved was an oxidation product of thiosulphate, possibly tetrathionate or one of the other polythionates. These compounds have been reported to be intermediate oxidation products of thiosulphate by the Thiobacilli 2.