When Fe2+ and S were supplied, T. ferrooxidans started growth by oxidizing Fe2+ in a well-aerated medium. After Fe2+ depletion, a lag phase followed before S oxidn. started. During S oxidn. at pH 95%. The no. of viable cells did not increase. Thus, resting cells of T. ferrooxidans which oxidize S to maintain their proton balance, reduce Fe3+ to Fe2+. The Fe2+-oxidizing system seemed to be inhibited at pH <1.3. At pH 1.8, Fe2+ was reoxidized at once. A scheme for the linkage of Fe and S metab. is discussed.