The ability of sulphate-reducing bacterial biofilms to reduce hexavalent chromium (Cr(VI)) to insoluble Cr(III), a process of environmental and biotechnological significance, was investigated. The reduction of chromate to insoluble form has been quantified and the effects of chromate on the carbon source utilization and sulphate-reducing activity of the bacterial biofilms evaluated. Using lactate as the carbon/energy source and in the presence of sulphate, reduction of 500 mu mol l(-1) Cr(VI) was monitored over a 48-h period where 88% of the total chromium was removed from solution. Mass balance calculations showed that ca 80% of the total chromium was precipitated out of solution with the bacterial biofilm retaining less than 10% of the chromium. Only ca 12% of the chromate added was not reduced to insoluble form. Although Cr(VI) did not have a significant effect on C source utilization, sulphate reduction was severely inhibited by 500 mu mol(-1) Cr(VI) and only ca 10% of the sulphate reducing activity detected in control biofilms occurred in the presence of Cr(VI). Low levels of sulphide were also produced in the presence of chromate, with control biofilms producing over 10-times more sulphide than Cr(VI)-exposed biofilms. Sulphide- or other chemically-mediated Cr(VI) reduction was not detected. The biological mechanism of Cr(VI) reduction is likely to be similar to that found in other sulphate-reducing bacteria.