A study on the citric acid production was performed in various size dual hollow fiber bioreactors with immobilized $\underline{Aspergillus niger}$ (KCTC 1232). The final dry cell mass density reached at 300 g/L based on the space volume available for cell growth. The pH of medium was a critical factor and the lower value resulted in higher citric acid yield. As the feeding rate of medium or the number of reactor unit was increased, the productivity was improved continuously due to higher consumtion rate of substrate. Under air and pure oxygen aeration the volumetric productivity reached at 0.63 and 1.02 g/L.h, which corresponded to 10-and 16-fold over those of batch fermentation, respectively.
