In this study, we investigate exopolysaccharide production by a bacterial consortium during the bioleaching of a cobaltiferrous pyrite. Whereas comparable studies have looked at exopolysaccharide production in batch systems, this study focuses on a continuous system comprising a series of four stirred bioreactors and reveals the difficulties in quantifying biomolecules in complex media such as bioleached samples. We also adapted the phenol/sulphuric acid method to take into account iron interference, thus establishing a new protocol for sugar quantification in bioleached samples characterised by low pH (1.4) and high iron concentration (2 g l(-1)). This allows sugar analysis without any prior sample preparation step; only a small amount of sample is needed (0.5 ml) and sample preparation is limited to a single filtration step. We found that free exopolysaccharides represented more than 80% of the total sugars in the bioreactors, probably because stirring creates abrasive conditions and detaches sugars bound to pyrite or bacteria and that they were produced mainly in the first two reactors where bioleaching activity was greatest. However, we could not establish any direct link between the measured exopolysaccharide concentration and bioleaching activity. Exopolysaccharides could have another role (protection against stress) in addition to that in bacterial attachment.