Thiomonas intermedia K12, a moderately acidophilic bacterium, which oxidizes S compds., exhibited the capability to use tetrathionate under oxic and anoxic conditions. Whereas under oxic conditions, the reduced S tetrathionate compd. was oxidized, under anoxic conditions, the organism disproportionated the compd. In both cases, trithionate and sulfate were produced but in different amts. The results of the tetrathionate degrdn. expts. under oxic conditions pointed towards a cyclic degrdn. process with a transient formation of trithionate and sulfate as the final products, similar to the mechanism described for acidophilic S compd. oxidizing bacteria. The results of the tetrathionate degrdn. expts. under anoxic conditions hinted to a partial redn. of tetrathionate to thiosulfate and a fractional oxidn. to trithionate and sulfate. In this study 4M tetrathionate were converted to 6M thiosulfate, 1M trithionate, 1M sulfate, and 8M protons. The DG0' of this reaction was -16.1 KJ/mol tetrathionate degraded. T. intermedia K12 grew under anoxic conditions with tetrathionate as the sole energy source. The cell nos. increased from 105 as the start value to 107/mL at the end. Org. compds., excluding traces of yeast ext., did not enhance growth. Therefore, it is proposed that tetrathionate disproportionation is a novel lithotrophic metab., which allowed T. intermedia K12 to survive changing conditions of O supply in S-compd.-rich environments and even to grow during this reaction. The extensive S compd. anal. was carried out by ion-pair chromatog.