Thiobacillus Ferrooxidans bacteria are commonly used for bio-leaching and recently in biobeneficiation of sulfide minerals. In this study, the effect of Thiobacillus Ferrooxidans on the froth flotation behavior of Sarcheshmeh copper ore in Iran has been investigated. Pure strains of Thiobacillus Ferrooxidans were used to promote surface chemical changes in pyrite and chalcopyrite, and thus influence their flotation behavior. In the presence of Thiobacillus Ferrooxidans, and xanthate as collector, pyrite was depressed, whereas chalcopyrite and other sulfide minerals were unaffected at natural pH. The present investigation shows that the surface chemical properties of bacteria can be manipulated successfully to achieve the desired effects on the flotation process. The results showed that recovery of pyrite in the presence of a depressant (Thiobacillus Ferrooxidans) is 50% lower than in the absence of any bacteria, demonstrating the depressing effect of bacteria on pyrite. It was concluded that the use of Thiobacillus Ferrooxidans would decrease the recovery of pyrite but would not changed the floatability of chalcopyrite. (C) 2004 Elsevier Ltd. All rights reserved.