The bioleaching of a nickel concentrate and an acid-consuming nickel ore was studied using a co-culture of Acidithiobacillus ferrooxidans and A. thiooxidans, as well as a thermophilic enrichment culture, VS2. The VS2 was dominated by a Sulfolobus species related to Sulfolobus metallicus. Nickel concentrate was readily solubilized with A. ferrooxidans and the VS2, resulting in nickel yields of 56% and 100%, respectively. Low-grade nickel ore required 350 g H2SO4 kg(-1) ore for maintaining the pH of the leaching solution below 3. To overcome the high acid demand, biological elemental sulfur oxidation was combined with the ore leaching. Leaching of a 2% (wt/vol) nickel ore with a co-culture of A. thiooxidans and A. ferrooxidans resulted in nickel yield of up to 86% with acid supplementation of 290 g H-2 SO4 kg(-1) ore. When coupled with biological sulfur oxidation, an 86% nickel recovery was achieved with 0.5% (wt/vol) ore concentration without further sulfuric acid addition. The VS2 oxidized sulfur at a rate of 0.063 g L-1 d(-1) and the simultaneous nickel ore leaching resulted in 100% nickel yield. In summary, the potential of using elemental sulfur addition and subsequent biological acid generation to maintain the low pH during bioleaching of an acid-consuming nickel ore was demonstrated. (c) 2005 Society of Chemical Industry.