The cells of Thiobacillus ferrooxidans were rapidly adsorbed on the solid surfaces of an agitated flask containing 1% pulp density of pyrite particles. More than ca. 99% of the inoculated cells were adsorbed. However, considerably fewer cells were adsorbed on pyrite particles than on the glass wall of the flask. Scanning electron microscope observation revealed that T. ferrooxidans cells were adsorbed aggregatively on restricted areas of the pyrite particles. The surfaces of the pyrite particles were characteristically eroded to show etched polyhedral pits, but without prominent cell adsorption of the extensively eroded surfaces during markedly enhanced leaching. When T. ferrooxidans cells were adsorbed on the solid surfaces, the iron-oxidizing activity of the bacteria was strongly inhibited, resulting in a failure to enhance pyrite oxidation. Adsorbed cells did not proliferate on the solid surfaces. When the adsorbed cells were released into an aqueous phase by the addition of the surface active agent Tween 20, the bacterial iron-oxidizing activity inhibited by adsorption was recovered and, as a result, pyrite oxidation was markedly promoted. Significant enhancement of pyrite oxidation by T. ferrooxidans was ascribed to the development and iron-oxidizing activity of the freely dispersed cells in an aqueous phase. The function of the surface active agent is to prevent tenacious adsorption of the bacterial cells to solid surfaces and organic substances examined, such as protein, nucleic acid, yeast extract, peptone, and cell free extracts, operate in the same way as the surface active agent. The enhancement of the bacterial pyrite oxidation by the intact cells of Thiobacillus thiooxidans is thought to be attributable to the organic substances excreted from T. thiooxidans cells and/or to the exchange adsorption of cells of both thiobacilli. The present results indicate that bacterial concentration in an aqueous phase rather than on pyrite particles plays major role in the enhanced oxidation of pyrite by T. ferrooxidans and that the bacteria contribute indirectly to pyrite oxidation through the regeneration reaction of ferric iron.