Aspergillus niger SZ was grown under citric acid accumulation conditions in a 11 bioreactor. Radioactive citric acid was added during the fermentation to the culture medium. The accumulation of radioactive citric acid in the mycellium was measure by measuring radioactive CO2 in the offgas. Evidence was obtained of significant turnover and metabolism of exogenous citric acid even during the phase of maximum accumulation into the medium.