Thiobacillus/errooxidans originally cultured on ferrous iron can adapt to grow on thiosulphate, tetrathionate, trithionate or sulphur in liquid media. In unneutralized cultures (initial pit 4--4.4),mean doubling times on thiosulphate were 8.5--13 h and on tetrathionate was about 10 h. In pH-control!ed cultures, growth on thiosulphate commenced at pH values from pH 3.6--4.7, with no growth in media initially below pH 3.5 or above pH 4.8. On tetrathionate, growth commenced between pH 1,5--4.3, with no growth below pit 1.3 or above pH 4.4. Growth rate was maximal (~ ~ 0.043 h -1) on thiosulphate a~ pH 3.6 and on tetrathionate at pit 2.5 (~ = 0.05 11-1). Growth yields of about 5.9 g (210 mmoles COg fixed) and 9.8 g (350 mmoles CO 2 fixed) dry wt per gram-molecule were obtained respectively on thiosulphate and tetrathionate. Bacteria. grown on thiosulphate or tetrathionate could produce colonies on ferrous iron agar, but this ability declined on prolonged culture on sulphur compounds. All the cultures grown on various inorganic sulphur compounds were capable of re-adapting to growth on iron. This was investigated in detail. About 66--820/0 of iron-grown organisms readily produced colonies on tetrathionate, but only about 1 in 109 produced colonies on thiosulphate.