Iron-oxidising bacteria, notably Thiobacillus ferrooxidans and Leptospirillum ferrooxidans, are capable of oxidising sulphide minerals either directly at the point at which the bacteria are attached to the mineral, or indirectly by the production offerric ions in the solution which then leach the mineral. In this study, the attachment of a mixed culture of these bacteria to a coupon of pyrite was studied. The bacteria form biofilms that are between 30 and 50 #m thick. In the earlier stages of biofilm growth, the coverage of the surface of the pyrite by the bacteria was not uniform; instead, clusters of microcolonies were separated by water channels. After ten to twelve days, a significant amount of a ferric oxide corrosion product formed on the surface of the pyrite, and the coverage of the surface by the biofilm was more uniform with a thickness of about 25 #m. During this stage of growth, the bacterial population was more concentrated at the solution interface. This work has shown that the attachment of these bacteria to the pyrite surface can occur as a biofilm, and the results suggest that the individual bacterium does not need to be attached to the pyrite for it to harness the mineral as an energy source. It is proposed that one of the functions of the biofilm may be to increase the concentrations offerrous andferric ions within the biofilm compared to the bulk solution. This hypothesis may be tested by using a microelectrode to determine the redox potential within the biofilm. Copyright ©1996 Elsevier Science Ltd