Acidithiobacillus ferrooxidans was immobilized in poly(vinyl alcohol) (PVA) by a PVA-boric acid method, and spherical beads of uniform size were produced. Biooxidation of ferrous iron by immobilized cells was investigated in repeated batch culture and continuous operation in a laboratory scale packed-bed bioreactor. During repeated batch culture, the cell-immobilized gels were stable and showed high constant iron-oxidizing activity. In continuous operation in a packed-bed bioreactor, biooxidation of ferrous iron fits a plug-flow reaction model well. A maximum Fe2+ oxidation rate of 1.89g l(-1) h(-1) was achieved at the dilution rate of 0.38h(-1) or higher, while no obvious precipitate was detected in the bioreactor. (C) 2003 Elsevier Ltd. All rights reserved.