Extractive citric acid fermentation with Aspergillus niger microoganism was investigated in the presence of corn oil and Hostarex A327 in oleyl alcohol. In the extractive fermentation, production and separation phases are achieved simultaneously. Before Response Surface Methodology (RSM) was applied,the effect of potassium ferrocyanide (K4Fe(CN)(6)) concentration on citric acid production and the prevention of solvent toxicity using corn oil were investigated. Citric acid concentration increased with increasing K4Fe(CN)(6) concentration. Solvent toxicity was reduced using corn oil in the extractive citric fermentation, In the RSM, the potassium ferrocyanide concentration, (0.02-0.10%, w/v), the ratio of organic phase volume to the aqueous phase volume, V-or/V-aq, (0.5-1.5), the corn oil concentration (0-20%, by volume) and the Hostarex A327 content in oleyl alcohol (10-50% by volume) were the critical components. Optimum citric acid concentration was obtained as 5.70 g/l with employing the following predicted optimum extraction conditions: K4Fe(CN)(6) of 0.06% (w/v), V-or/V-aq of 1.25. corn oil concentration of 13.7% (by volume) and Hostarex A327 content in oleyl alcohol of 33% (by volume). Citric acid production by extractive fermentation was increased approximately 40% with respect to the control run (4.10 g/l). (C) 2002 Elsevier Science Ltd. All rights reserved.