Citric acid production by Aspergillus niger immobilized in porous cellulose carriers, commercially known as MICROCUBE, was investigated at different volume ratios of carriers. MICROCUBE was prepared using viscose to immobilize A. niger for culture. The result showed that the acid productivity in the immobilized culture was twice that in a conventional suspension culture. In addition, it was found that the acid productivity depended on carrier volume ratio, where a maximum productivity of 8.9 g/l.d was obtained at 30% v/v. At this volume ratio, repeated batch cultures were tested at different time intervals of 5, 7 and 9 d. In all cases, the activity of immobilized A. niger was maintained for up to 40 d a maximum amount of acid was obtained at 7-d batch interval.