The present study deals with the effect of copper ions on mould morphology and citric acid productivity by Aspergillus niger GCB-47 and its mutant derivative GCMC-7, using 150 g/l carbohydrates of cane-molasses as the basal fermentation media. The mutant showed a 1.26-fold increase in citric acid production over the control and 2.82 times the wild-type culture. The addition of 2.0 x 10(-5) M CuSO4 to the fermentation medium reduced the Fe2+ ions concentration by counteracting its deleterious effect on fungal growth. The copper ions also induced a loose-pelleted form of growth (0.5 mm, diameter), reduced the biomass concentration (12.5 g/l) and increased the volumetric productivity of citric acid monohydrate (92.0 +/- g/l). On the basis of comparison of kinetic parameters, namely the volumetric substrate uptake rate (Qs) and specific uptake rate (qs), the volumetric productivity, theoretical yield and specific product formation rate, it was observed that the mutant was faster growing organism (Yx/s = 0.938 +/- 0.03 g/g) and had the ability to overproduce citric acid (Yp/s = 0.938 +/- 0.03 g/g). (C) 2002 Elsevier Science Ltd. All rights reserved.