An external-loop airlift bioreactor (11.5 l), with a ratio of height to diameter of the riser of 2.9, was used for the production of citric acid from the mash of dried sweet potato with its dregs by Aspergillus niger. The effects of air flow rate, liquid volume and sparger hole diameter on citric acid production were investigated. A comparison of citric acid fermentation was made between the external-loop airlift bioreactor and the mechanically stirred tank bioreactor. After a total fermentation time of 65 h, an average of 10.6 mg/ml citric acid concentration was obtained in the external-loop airlift bioreactor under an air flow rate of 1.3 vvm, liquid volume of 8.5 l as compared to 9.6 mg/ml citric acid concentration in the 10 l mechanically stirred tank bioreactor under optimised operation conditions: agitation rate 200 rpm, air flow rate 1.0 vvm and liquid volume 6.5 l. It was demonstrated conclusively that the external-loop airlift bioreactor could be used to produce citric acid economically using A. niger from the mash of dried sweet potato with its dregs.