CO2 supply at 4% (v/v) in air increased the biooxidation of a gold concentrate (41% enargite, 43% pyrite) with Thiobacillus ferrooxidans in a 2-l bioreactor at 4% (w/v) solids concentration, 35 degrees C and pH 2.4. Extraction increased from 21 to 69% for Fe, 19 to 25% for As, and 16 to 19% for Cu. Suspended biomass increased from 2.6.10(7) to 1.2.10(8) cells/ml. It is concluded that the biooxidation of this gold concentrate is limited by the rate of CO2 supply.