The microbial communities were profiled by polymerase chain reaction (PCR) ¡ª denaturing gradient gel electrophoresis (DGGE) followed by partial sequencing of 16S rRNA gene. Large temperature gradients prevailed resulting in the simultaneous presence of active mesophilic and thermophilic iron- and/or sulfur-oxidisers in the heap. As mineral oxidation progressed microbial diversity decreased and Acidithiobacillus ferrooxidans became increasingly dominant. The number of bacteria in the leach liquors was in the range of 105-107 cells mL? 1. After one year of bioheap operation several ore samples were drilled from the heap and A. ferrooxidans, Acidithiobacillus caldus, an uncultured bacterium clone H70 related organism, Ferrimicrobium acidiphilum and a bacterium related to Sulfobacillus thermosulfidooxidans were found. Cell counts from the ore samples varied between 105 and 107 cells g? 1 ore sample. The archaeal species present in leach liquors were novel and related to uncultivated species. During the secondary leaching phase the leaching community remained steady. A. ferrooxidans dominated, and an uncultured bacterium clone H70-related organism and Leptospirillum ferrooxidans were present.