The use of magnetic rotating biological contactor (MRBC) was introduced in citric acid fermentation. Two types of contactors, disk and drum systems, were used. The effect of magnetite (Fe3O4) on citric acid fermentation in the surface culture method was examined. Dissolution of magnetite during fermentation did not show major adverse effects. Batch fermentation using the disk and the drum showed the maximum citric acid concentration of 68.9 g . l(-1) and 73.1 g . l(-1) at the period of 31 and 36 d, respectively. In the disk system, a specific biofilm area of 4.2 cm(-1) against working volume was found to be effective, while 1.8 cm(-1) was observed in the drum system, yielding higher citric acid production. A combination of an 8 d period of acclimatization and 2 ml . h(-1) flow rate was a suitable condition far continuous citric acid fermentation. In the drum system, citric acid was produced with a higher productivity (4.3 g . l(-1) . d(-1)) compared with that in the disk system (2.2 g . l(-1) . d(-1)) and biofilm capable of maintaining its stable activity for about 50 d of continuous operation.