The production of gibberellic acid (GA(3)) by submerged cultivation of Gibberella fujikuroi was carried out in a 3 1 fermenter. Under optimum culture conditions, the batch culture exhibited a maximum biomass concentration of 12 g/l and GA(3) production of 1 g/l in 170 h. An unstructured mathematical model was proposed to describe the batch kinetics of fermentation and the model parameters were determined using batch kinetic data. The model was extrapolated to computer simulation and design (offline on computer) of nutrient feeding strategies for fed-batch cultivation for production of high concentration and productivity of GA(3). Using this model-based nutrient feeding strategy; a volumetric productivity of 0.0168 g/l h) for GA(3) was obtained, which was approximately 2.9 times higher than that obtained in conventional batch cultures. (c) 2004 Elsevier Ltd. All rights reserved.