Batch fermentation of glucose to gluconic acid was conducted using Aspergillus niger under growth and non-growth conditions using pure oxygen and air as a source of oxygen for the fermentation in 2 and 5 1 stirred tank reactors (batch reactor). Production of gluconic acid under growth conditions was conducted in a 51 batch reactor. Production and growth rates were higher during the period of supplying pure oxygen than that during supplying air, and the substrate consumption rate was almost constant. For the production of gluconic acid under non-growth conditions, conducted in the 21 batch reactor, the effect of the pure oxygen flow rate and the biomass concentration on the gluconic acid production was investigated and an empirical equation suggested to show the dependence of the production rate r(p) on the biomass concentration C-x and oxygen flow rate Q, at constant operating conditions (30 degreesC, 300 rpm and pH 5.5). Biomass concentration had a positive effect on the production rate r(p), and the effect of Q on r(p) was positive at high biomass concentrations. (C) 2003 Elsevier Ltd. All rights reserved.