Aspergillus niger cells immobilized in agarose beads were utilized for the production of citric acid from soy whey, a by-product generated during the tofu-making process. Soy whey samples supplemented with 10% sucrose were inoculated with 10% (w/v) free and immobilized cells and incubated at 30 degrees C in a shaker water bath at a speed of 200-220 rpm. Maximal citric acid yields of 21 g/l and 27 g/l with free and immobilized cells, respectively, were recorded on the 10th day under repeated batch conditions.