The effect of cinnabar on pyrite oxidation by mercury-sensitive and mercury-resistant strains of Thiobacillus ferrooxidans was investigated by using percolation columns. Mercury-resistant strains oxidized pyrite in pyrite-cinnabar mixtures (1 and 10%, wt/wt), whereas a mercury-sensitive strain did not. Elemental mercury was produced by the mercury-resistant strains growing in the pyrite-cinnabar mixtures in percolation columns and in flasks containing cinnabar only. Manometric experiments showed that cinnabar had little effect on oxygen uptake of mercury-sensitive or mercury-resistant cells growing on ferrous sulfate, pyrite, or pyrite-ferrous sulfate mixtures. In addition, shake flask leaching experiments showed that cinnabar had little effect on pyrite oxidation at 1% (wt/wt) but inhibited growth of mercury-sensitive and mercury-resistant strains at 10%. Mercury-resistant strains were unable to grow on cinnabar as an energy source.