Thiobacillus ferrooxidans was immobilised on sand (size 0.85 mm to 1.18 mm) for use in a repeated batch and continuously operated packed-bed bioreactor which has not been previously reported in the literature. Repeated batch operation resulted in the complete oxidation of ferrous to ferric iron. The bacteria were active immediately after 3-4 weeks in a non-aqueous medium; i.e. the sand was allowed to dry out, demonstrating the stability of the system. A lag phase of 28 days was recorded when the sand was stored dried in a sealed container for 16 weeks compared with a lag phase of 13 days for a sample frozen for 18 weeks. After a period of 10 days, continuous operation of the reactor at a dilution rate of 0.64 h(-1) resulted in 95-99% oxidation of ferrous iron or 0.31-0.33 kg m(-3) h(-1). With the use of a scanning electron microscope, images were recorded of Thiobacillus ferrooxidans on sand.