The influence of Thiobacillus ferrooxidans bacteria on efficiency of nickel and cobalt leaching from natural pentlandite (Ni,Fe)₉S₈ have been examined. The effect of pulp density particle size and initial ferrous ion concentration in leaching solution on the leaching yield was also evaluated. It has been demonstrated that the presence of Tferrooxidans bacteria in leaching system induced considerable increase in nickel and cobalt leaching from pentlandite. The decrease in efficiencies of nickel and cobalt bacterial leaching processes have been denoted in systems containing high amounts of ferrous ions. Initial Fe²⁺ concentration of 4.5 g/dm³ bioleaching. The 20% pulp density may be admitted to be optimum regarding both satisfactory yields of bio leaching process and high nickel and cobalt concentrations in leaching solutions. The obtained results showed that particle size of 90-125 urn was optimum for nickel and cobalt bioleaching from