A new mutant strain, Aspergillus niger GS-III, showing resistance to manganese ions inhibition of citric acid fermentation on a sugarcane molasses containing medium was induced from Aspergillus niger KCU 520, a high citric acid-yielding strain. In submerged, surface or continuous cultures in the presence of manganese ions concentration upto 1.5 ppm the mutant strain yielded citric acid about 90 KgM(-3). The citric acid yield was comparable to that obtained with the parental strain KCU 520 in the absence of manganese ions, but it was at least 3-fold higher than that obtained by : the latter in the presence of manganese ions. The mutant strain immobilized in calcium alginate beads was used in combination with surface-stabilized cultures for about 36-days in a continuous flow horizontal fermenter without any apparent loss in citric acid productivity. These results indicate that the manganese-resistant mutant is stable and may be used in the presence of sufficient manganese ions concentration (1.5 ppm) in the fermentation medium. This capability of the mutant strain A. niger GS-III has been correlated with greatly reduced levels (about one-thirds) of the NADP(+)-isocitric dehydrogenase, one of the control points for citric acid accumulation.