High temperature biooxidation of ferrous iron was studied in a batch system, using the acidophilic thermophile Acidianus brierleyi. The effect of ferrous iron initial concentration on the growth and activity of the cells was investigated A. brierleyi was able to grow on ferrous iron at concentrations below 7.5 kg m(-3). The values of specific growth rare and yield were 0.043 h(-1) and 2.2x10(14) cells/kg iron respectively. At ferrous iron concentrations of 7.5 kg m(-3) and higher the growth of the cells was prohibited, however the non-growing cells were able to oxidise iron. The maximum biooxidation rate of ferrous iron, 0.105 kg m(-3) h(-1), was achieved in a culture initially containing 7.5 kg m(-3) of Fe2+. The mesophilic iron oxidiser Thiobacillus ferrooxidans was capable of growing on ferrous iron at concentrations as high as 30 kg m(-3). Moreover the rate of mesophilic biooxidation of ferrous iron was significantly higher than that observed in the presence of A. brierleyi.