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Studies in conventional batch culture confirmed that initial glucose concentration in the fermentation medium affected both the rate of citric acid fermentation by Aspergillus niger and the morphology of the producer micro-organism. A series of fed-batch (glucostat) experiments, in which the concentration of glucose was kept constant whereas all other conditions were allowed to change, were used to eliminate the effects of decreasing sugar concentration found during conventional batch experiments. The level of glucose had a marked effect on production rates; the specific rate of citric acid formation increased with increasing initial glucose concentration (batch culture) or glucose levels (glucostat culture), and in both culture methods, the specific growth rate increased with decreasing glucose concentration for the first 48 h of fermentation. The reduction observed in the mean length of filaments at low glucose levels …