The present study deals with the direct production of citric acid from raw starch by Aspergillus niger. Shake flask and semi solid culture methods were compared using A. niger GCB-47 (parental strain) and GCMC-7 (mutant strain). When cultivated in shaking culture with 150 g/l soluble starch as a carbon source, the mutant strain GCMC-7 produced 69.5 g/l citric acid, which was, 1.48-fold greater than the parental strain GCB-47. From a practical viewpoint, direct production of citric acid from corn and potato starch was examined using semi-solid culture. On the basis of a comparison of kinetic parameters namely the volumetric substrate uptake rate (Qs), the specific substrate uptake rate (qs), the volumetric productivity, theoretical yield and specific product formation rate, it was observed that the mutant strain was a faster growing organism. The mutant strain GCMC-7 produced 71.4 and 92.9 g/l citric acid, approximately 1.44 and 1. 12 times as much as the parental strain GCB-47, from 200 g/l corn and potato starch, respectively. The findings suggest that GCMC-7 possesses enhanced ability for sugar metabolism and citric acid production. (C) 2002 Published by Elsevier Science Ltd.