The ability of the chemolithotrophic bacterium Thiobacillus ferrooxidans to oxidise ferrous sulphate has been exploited in the process of bioleaching, treatment of acidic mine drainage and removal of H2S from sour gases. Over the years, much attention has been paid to this organism and the process it catalyses. Unfortunately, inconsistent and ambiguous results have been reported in the literature. Therefore, further studies need to be undertaken in order to verify the optimal pH and temperature ranges of 2.0-2.3 and 30-33 degrees C respectively and the optimal levels of O-2 and CO2 as well as the values of the basic kinetic parameters and the effect of substrate and product inhibition. On the process side, the work has mainly been aimed at improving the rate of ferrous iron oxidation and numerous immobilisation matrices and bioreactor systems have been tested.