Interaction of Acidithiobacillus thiooxidans and Acidithiobacillus ferrooxidans with soluble uranium(VI) was studied. Acidithiobacillus thiooxidans aerobic cultures showed considerable decrement of uranium in solution when pH was set to 4 with KOH. This condition stabilized polythionates produced as a result of the oxidative sulphur metabolism and favoured uranium indirect reduction. Acidithiobacillus ferrooxidans was able to use uranium(VI) as an alternative final acceptor in the respiratory chain when elemental sulphur was the electron donor. Bacterial growth was also supported by this process obtaining bacterial yields comparable with those when iron(III) was the final acceptor.