The effect of increased levels of dissolved carbon dioxide (dCO(3)) on batch cultures of Aspergillus niger A60 was examined by means of continuous gassing with inlet gas mixtures containing a range of CO2 concentrations (0, 1, 3, 5, 7.5, 12, 15% v/v). In addition to assessing the effect of elevated elevated dCO(2) on the process in terms of biomass, product (citrate) formation, and substrate consumption, the impact on the morphology of the fungus was quantitatively assessed by use of an image analysis system which was used to measure critical morphological parameters such as main hyphal length, mean branch length, and the value of the hyphal growth unit. In processes where the inlet gas stream contained more than 3% CO2, raised CO2 was associated with decreased biomass and citrate concentrations, decreased substrate consumption, and changes in yield factors. Morphologically, elevated dCO(2) was associated with changes in the value of the critical morphological parameters. This effect became more pronounced as the rime of exposure increased. Some possible mechanisms for these results are discussed including the possibility of specific effects on critical enzymes in the citrate synthetic pathway.