The Mouteh refractory pyrite concentrates at pulp densities of 1.5%, 3%, 4.5% and 6% were treated, using Thiobacillus ferrooxidans DSM 581 and the same bacteria adapted on the Mouteh pyritic concentrate. Compared with a non-adapted culture, use of an adapted inoculum of T. ferrooxidans increased bioleaching rate of iron by a factor of 1.940, 2.011, 1.859 and 1.559 for pulp densities 1.5%, 3%, 4.5% and 6%, respectively. Lag phase time for growth of adapted cells decreased to less than 24 hours. Ore samples were analysed for gold recovery by cyanide extraction before and after biooxidation in 4L and 20L bioreactors. When 55% of the sulphides were oxidised, as a result the gold recovery-upon subsequent cyanide extraction improved more than 95%. Mathematical analysis of bioleaching data showed that the X3 variable equation satisfactorily predicts the gold recovery in relation to the oxidation degree of pyrite concentrate.