Three strains of Thiobacillus ferrooxidans have been isolated from copper mine in Bulgaria, and investigated. The levels of their ferrous- and sulfur-oxidizing activities have been tested as well as their tolerance to copper and zinc ions. The effect of both metal ions on ferrous- oxidizing activity and oxygen uptake have been investigated too. A possible mechanisms for the tolerance of these bacteria to Zn2+and Cu2+ions is discussed.