The oxidation of uranous compounds by washed cell suspensions ofThiobacillus ferrooxidansandT. acidophiluswas monitored with a Clark oxygen electrode. The bacteria were able directly to oxidize U‐IV, which was supplied as uranous sulfate or as uranous oxide. The rates of oxygen uptake were dependent on: the cell concentration; the previous growth history of the organisms; the amounts of U‐IV and of inhibitors; pH and the presence of iron sulfates.