A sample of black shale fraction of copper ore from the Lubin deposit, Poland, was subjected to pretreatment by different microorganisms to improve the copper recovery during the subsequent flotation. Chemolithotrophic bacteria grown on So and S2O3 2- were used to depress pyrite before flotation by means of ethyl xantate as collector. Sulphate-reducing bacteria were used to perform a prior sulphidization of the black shales before flotation by means of the same collector. Two-stage flotation experiments were carried out in which initially the gangue minerals of the ore were depressed by means of “silicate” bacteria and then, during the flotation of the non-floated product from the first stage, the pyrite was depressed by means of pretreatment with So- grown chemolithotrophs. In all these cases the microbial pretreatment had positive effect on the copper recovery.