The effect of solid loading (mineral pulp density) on thermophilic bioleaching of pyrite by Sulfolobus metallicus (BC) was investigated in a batch reactor. Different mineral pulp densities in the range 3-18% (w/v) were tested. With mineral pulp densities ranging from 3 to 9% the bioleaching proceeded in a single stage with a relatively constant rate. The bioleaching rates calculated for pulp densities of 3, 6 and 9% were 0.10, 0.11 and 0.09 kg iron m(-3) h(-1) respectively. By contrast the bioleaching of pyrite at pulp densities of 12 and 15% proceeded in two distinct stages. During the exponential phase of microbial growth a sharp and linear increase in concentration of released iron was achieved. This increasing trend levelled off in the presence of non-growing cells and the second stage of bioleaching continued with a slower rate. For the pulp density of 12% the bioleaching rates of the first and the second stages were 0.09 and 0.02 kg iron m(-3) h(-1) respectively, whereas the calculated rates in the presence of 15% mineral were 0.07 and 0.017 kg iron m(-3) h(-1) for the first and the second stages. Application of 18% mineral adversely influenced the activity of the cells and the extent of bioleaching in this case was insignificant.