Ferrous iron oxidation by Thiobacillus ferrooxidans was studied in shake flasks and a bubble column render different aeration conditions. The maximum biooxidation rare constant was affected by oxygen transfer only at low aeration intensities. At oxygen transfer rates higher than 0.03 mmol O-2 l(-1) min(-1), the maximum biooxidation rate constant was about 0.050 h(-1) in both shake flasks of different size and the bubble column. The oxygen transfer rate could be used as a basis for scaling up bioreactors for ferrous iron biooxidation by T. ferrooxidans.