A direct relationship between jarosite precipitation and the number of attached cells were found in Thiobacillus ferrooxidans cultures in the presence of glass beads. Low cell immobilisation was observed on the glass beads; these cells were probably attached to the jarosite deposits on the glass. The presence of glass beads only contributed to biofilm formation increasing jarosite precipitation especially at higher pH values. At a pH value lower than 1.6, a biofilm was not produced due to negligible iron precipitation. Cultures with an initial pH value of 1.8 reached an iron(III) productivity as high as 15 mM h(-1) after five successive cultures obtained by transferring previous solid residues to new media. (C) 2000 Elsevier Science Ltd. All rights reserved.