Thiobacillus ferroDxidans and thiobacillus thiooxidans have a main effect on the oxidation of copper sulfide ores. Properties of water percolated through the bed of the Open Pit of Erdenet Mining Corporation are different from that of surface water by its chemical composition, which creates a favorable environment for the life of thiobacillus. However, number and activeness of thiobacillus in percolated water do not reach a sufficient level to use them directly as an oxidizer of sulfides in bioleaching practice.It is possible to determine optimal conditions for reproduction of thiobacillus through an experiment and to increase the number ofthiobacillus in percolated water up to 45 x 1(j8 -70 x 10 10 celVml or 2.5-3 x 1()6 times.