Sulphate was rapidly bound by cell suspensions of Thiobacillus ferrooxidans. The binding was depressed by tetrathionate but was unaffected by Group VI anions, cysteine or methionine. Increasing uptake of sulphate was observed in cell suspensions incubated in the presence of ferrous iron. The bulk of 35S-sulphate was removed from the organisms by washing with dilute sulphuric acid and the remaining label was incorporated into cold trichloroacetic acid-soluble compounds. 35S-labelled adenosine 5'-sulphatophosphate was produced from ATP and 35S-sulphate by cell suspensions and in cell-free extracts. There was no evidence for the production of adenosine 3'-phosphate 5'-sulphatophosphate assayed by a very sensitive bioluminescence method.