The rate of iron (II) oxidation by sulphur-grown Thiobacillus ferrooxidans cells decreased when the pH of the original growth medium was lowered. This behaviour was observed even after shifting from the original growth pH to a higher pH. After being suspended in medium at a pH higher than the growth pH, sulphur-grown cells could leach covellite at a similar initial rate to iron-grown cells. Sulphur-grown cells exhibited a long lag phase when the original growth pH was low. These results were correlated with the number of protons associated with the cell surface, rather than with cell hydrophobicity or cell capacity to attach to solid particles. Sulphur-grown cells grown in very acidic media (without pH control) were not able to oxidize iron (II) or leach covellite even after shifting to a high pH.