The optimisation of gluconic acid fermentation using immobilized Aspergillus niger on a highly porous cellulose support is described. Experimental results showing the effects of variations in oxygen partial pressure, glucose concentration and biomass concentration have been obtained with a continuous recirculation reactor. Levels of dissolved oxygen and glucose concentrations during fermentation significantly affect the production and fermentation time. The optimum biomass requirement on a porous cellulose support has been estimated to be 0.234 mg cm(-2) for efficient bioconversion. Increasing the quantum of biomass beyond this value resulted in an overgrown biofilm, which affected productivity adversely. Morphological characteristics of immobilized A. niger have also been investigated. (C) 2002 Elsevier Science Ltd. All rights reserved.