Response-surface experiments with the Box-Behnken design were conducted on Aspergillus foetidus CCRC 30206 to determine the optimal concentrations of carbon, nitrogen and phosphorus for citric acid production. Results of six-day fermentations at 30 degrees C showed that the minimal contours of citric acid production under various sucrose concentrations (60-180 g/litre) overlapped at a region where the concentrations of ammonium chloride and potassium dihydrogen phosphate were in the range of 1.5-2.0 and 0.6-1.0 g/litre, respectively. Accumulation of citric acid occurred when growth was limited by the supply of either nitrogen or phosphorus. Nitrogen exerted a stronger catabolite repression on citric acid production than phosphorous. The maximal citric acid concentration and specific production rate appeared at the region where the respective concentrations of carbon, nitrogen and phosphorus sources were around 48, 0.50 and 0.35 g/litre. The optimal levels of carbon, nitrogen and phosphorus sources for a 15-day fermentation differed from those of a six-day fermentation.