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 could be explained by increased branching frequency as a result of the increasing specific growth rate at the early stages of fermentation. The size of the mycelial clumps was reduced at low glucose levels and their shape was also affected as the ratio between the perimeter of the clump and the perimeter of its core increased as glucose concentration decreased. Very low glucose levels during fermentation in glucostat culture had a striking effect on the metabolism of the micro-organism because the specific production rate decreased in favor of the specific growth rate and the morphology changed with a doubling of the size of clumps and pellet formation. The main morphological observations in both batch and glucostat cultures seemed to be related to the growth rate, which is restricted by the glucose concentration, and, thus, indirectly related to the concentration of glucose in the fermentation media. The glucose levels in the medium were shown to lend to a significant noncatalysed entry into the mycelium, which accounted for the citric acid productivity and growth rate changes.