The influence of aeration rate on Cordyceps militaris morphology and exo-biopolymer production was investigated in a 5-1 jar fermentor, The mycelial morphology of C, militaris was characterized by image analysis, which included mean diameter, circularity, roughness, and compactness of the pellets. Cells were observed to form mainly pellets during the entire culture period irrespective of aeration conditions. There existed a notable variation in morphological parameters between the pellets grown on different aeration conditions, by which exo-biopolymer production yields were correspondingly altered. The mean diameter and compactness of the pellets indicated higher values at 2 vvm (volume of air per volume of culture per minute). which was closely related to exo-biopolymer biosynthesis. The more compact pelleted form was favourable for exo-biopolymer production. Under extremely low and high aeration conditions (e.g. 0.5 and 4 vvm), severe deformations of pellets (autolysis of core and shaving off the outer hairy region) were observed at the later stages of fermentation associated with a decrease in morphological parameters, (C) 2002 Elsevier Science Ltd. All rights reserved.