Chemostat cultures Aspergillus niger A60 were aerated with mixtures containing elevated CO2 (2, 4, 7.5, 10 and 18% v/v). The effects of such treatment upon biomass, citrate, and gluconate steady-state values were quantified and the effect on key morphological indices (hyphal growth unit, mean main hyphal length, and mean branch length) was assessed using computerized image analysis techniques. These were compared with steady-state values fr om a control gassed solely with air. It was found that although elevated CO2 did inhibit growth and organic acid formation and generally led to an increase in the value of the morphological parameters, these effects were more modest that might have been expected based on results from previous batch studies on filamentous fungi. The possible mechanisms contributing to the observed inhibition and the reasons for the differences from previous studies are discussed.