Production of citric acid by Aspergillus niger was studied in a rotating biological contactor (RBC) consisting of plastic disks mounted on a horizontal shaft with polyurethane foam (PUF), as a porous biomass support, attached on each side of the RBC disks. Mycelia of A. niger formed the biofilm of immobilized cells on the surface of the PUF. The RBC-PUF system was operated with a rotational speed of 10 rev/min during the stage of biofilm formation. The mature biofilm was exposed to the fermentation medium and the air space alternately. The results showed that the volumetric productivity obtained with the RBC-PUF system (0.896 g/l h) was almost three times higher than that obtained with a stirred-tank fermenter (0.33 g/l h). The immobilized biofilm was active for over 8-cycle periods of citric acid production with repetitive use without loss of bioactivity.