PVA-cryogels entrapping about 10 9 cells of Acidithiobacillus ferrooxidans per ml of gel were prepared by freezing-thawing procedure, and the biooxidation of Fe2+ by immobilized cells was investigated in a 0.365 l packed-bed bioreactor. Fe2+ oxidation fits a plug-flow reaction model well. A maximum oxidation rate of 3.1 g Fe2+ l(-1) h(-1) was achieved at the dilution rate of 0.4 h(-1) or higher, while no obvious precipitate was determined at this time. In addition, cell-immobilized PVA-cryogels packed in bioreactor maintained their oxidative ability for more than two months under non-sterile conditions. Nomenclature: C-A0 - Concentration of Fe2+ in feed stream (g l(-1)) C-A - Concentration of Fe2+ in outlet stream (g l(-1)) D - Dilution rate of the packed-bed bioreactor (h(-1)) F - Volumetric flow rate of iron solution (l h(-1)) F-A0 -Mass flowrate of Fe2+ in the feed stream (g h(-1)) K - Kinetic constant (11(-1) h(-)1) r(A) - Oxidation rate of Fe2+ (g l(-1) h(-1)) V - Volume of packed-bed bioreactor (l) X-A - Conversion ratio of Fe2+ (%)