In this research work, selective depression of pyrite in the flotation of a low-grade lead-zinc ore containing 31% pyrite was investigated in the absence and presence of Acidithiobacillus ferrooxidans. Pyrite was significantly depressed with these bacteria using the optimum dosage of reagents. Sphalerite recovery and Zn grade in the obtained sphalerite concentrate were both enhanced by bacteria. The results of bioflotation experiments showed that bacterial depression of pyrite is very sensitive to the concentration of other flotation reagents. Least significance difference (LSD) bars were used to study the significance of the factors under study with a confidence interval of 90%. Under similar conditions,A.ferrooxidans was seen to increase the Zn grade and separation efficiency more effectively than sodium cyanide. Flotation kinetic studies confirmed a considerable decrease in the pyrite kinetic rate constant in the presence of A. ferrooxidans. (C) 2009 Elsevier Ltd. All rights reserved.