The effect of the concentration of ferrous iron on the rate of its oxidation was studied with Thiobacillus ferrooxidans. The specific rate of oxidation of ferrous iron was found to increase with its concentration and to decrease with a rise of the total iron content. Cultivation of bacteria with electrochemical reduction of Fe3+ produced high cell concentrations at a low content of total iron (4--6 g/litre), e. g. upto 4.5 g/litre (dry biomass weight).