Gluconic acid is produced by a fermentation process in which an inoculum of the fungus Aspergillus niger is transferred to a first production medium containing a source of glucose at a time when the glucose oxidase activity of the inoculum is increasing at its maximum rate. The fungus mycelium can be separated from the production medium at harvest time and reused in successive production media without the need for added nutrients until such time as nutrients are needed to revive the glucose oxidase activity.