This study reports on the effects of internal fermenter and external in-line agitation and fed-batch mode of operation on citric acid production from Candida lipolytica using n-paraffin as the carbon source. An optimum range of fermenter agitation speeds in the range 800-1000 rpm corresponding to Reynolds numbers of 50433-62947 (based on initial batch conditions) seemed to give the best balance between substrate utilization for biomass growth and citric acid production. Proof of concept evidence is presented that indicates that an external in-line agitator could be used in place of high speed internal agitation to increase citric acid production. However, more work is required to optimize the external agitator concept. Application of multiple fed-batch feedings can be used to extend the batch fermentation and increase final citric acid concentrations and product yield. Experiments were conducted implementing a three-cycle fed-batch process which increased overall citric acid yields to 0.8-1.0g citric acid g(-1) n-paraffin, approximately 200% improvement from those found in the normal batch process. The three-cycle fed-batch mode of operation also increased the final citric acid concentration to 42 g dm(-3) from about 6 g dm(-3) for normal batch operation. Increased citric acid concentrations in three-cycle fed-batch mode was achieved at longer fermentation times. (C) 2004 Society of Chemical Industry.