In this study, the effect of Ferric ion and cell concentrations on the oxidation of Ferrous ion by T. ferrooxidans was investigated. Ferric ions competitively inhibited ferrous ion oxidation by the bacteria. The inhibitory effect of ferric ion was, however, reduced by increasing cell concentration, The apparent ferric ion inhibition constant did not change with increasing cell concentration. The ferrous ion oxidation kinetics in the absence and presence of ferric ion changes from the standard Michaelis-Menten type at low cell concentrations to pseudo-first-order kinetics at high cell concentration.