It is known from nitrogen-limited batch cultures of Aspergillus niger that intracellular ammonium concentration is closely related to the specific oxygen uptake rate, especially during the idiophase of the culture. The responses of specific oxygen uptake rate to ammonium pulse feeding were examined at different levels of intracellular ammonium concentrations. A quantitative relationship between the specific oxygen uptake rate and the intracellular ammonium level was obtained and applied to the overproduction of citric acid.