Strain improvement of Candida oleophila C-73, a strain capable to produce 99.71 gram of citric acid per litre in PM3 medium, was performed by mutating with NTG for two consecutive steps and then with UV for the last step. Potency index, a technique developed in this study, was used for primary screening for the mutants. Secondary screening was done by determination of citric acid concentration on fermentation broth by pentabromoacetone and HPLC methods. With these techniques, the correlation between potency index and citric acid production was observed. Strain N-57, NN-1 and NNU-62 were mutants selected from each step of the treatment showing highest ability to produce citric acid of 120.54, 128.44 and 136.66 gram per litre, respectively after 96 hours of cultivation which were 20.89%, 28.81% and 37.06% higher than that produced by strain C-73. Cultivation of NNU-62 in 5-L fermentor gave maximum citric acid yield of 173.36 gram per litre after 96 hours of cultivation. Stability of citric acid producing ability of NNU-62 was still observed after 5 consecutive fermentations within 75 days.
