The process of biological oxidation of ferrous iron by the microorganism Leptospirillum ferriphilum was studied in different types of biofilm reactors. A total of 12 bioreactor types with both fixed and mobile support were used. The biofilm supports used had different shapes and were made of: polyethylene, nylon, polystyrene, polyvinylidene difluoride, polyester, ceramics, carbon and PVC. The pH of the microbial medium was maintained around 1.0, the temperature was 40°C while the influent ferrous iron concentration was 20 g/L. Iron oxidation rates of up to 4.5 g/L/h were obtained. The most efficient were the bioreactors with polyethylene and nylon woven fabric support. However, they oxidized ferrous iron at high rate for relatively short time periods - 40 to 60 days. While the bioreactor with a fixed bed of Raschig rings support had lower iron oxidation rate, their long-term stability was much higher. Of all the biofilm support materials tested, it was found that only polyvinylidene difluoride did not allow the formation of biofilm. Since no significant amounts of jarosite were formed at pH of 1.0 and below, the biofilm formed was very weak mechanically. For that reason the moving support, such as inverse fluidized bed, was not very appropriate because of the high shear stress.