Copper dissolution from a sulfide ore (with covellite as the main copper phase) was investigated in cultures ofThiobacillus ferrooxidansorThiobacillus thiooxidansand in abiotic controls. In unsupplemented media,T. ferrooxidanswas more efficient thanT. thiooxidans. In the presence of ferric iron, the dissolution of covellite was not significantly different in cultures inoculated withT. ferrooxidansorT. thiooxidans. However, the most extraction was found inT. thiooxidanscultures supplemented with ferrous sulfate. The first results were explained by the mechanism proposed by Schippers and Sand (Appl Envir Microbiol 65:319-321, 1999), which involves polysulfides and sulfur as intermediates. This mechanism was extended to explain the behavior ofT. thiooxidansculture supplemented with ferrous iron.