Batch cultures of Candida utilis CBS 621 were carried out in a pressurized reactor under increased air pressure up to 6 bar. The effect of total air pressure was also investigated in a high cell density fed-batch culture, raising the total air pressure from 1 bar to 12 bar. The results showed that the rise of air pressure, for both operation modes, led to a substantial enhancement of biomass production. Moreover, ethanol formation was significantly reduced at 6 bar and 12 bar air pressure, respectively for batch and fed-batch processes. A method using automatic image analysis for classification of C. utilis cells based on their morphology was developed and applied to experimental data. Morphological parameters such as single and budding cells, cell size and elongation factor, were analyzed to assess the pressure effect on yeast culture. No significant differences were observed in cell size distribution and yeast cells retained the typical oval form, even at 12 bar air pressure. In what biological aspects are concerned, it is possible to state that C. utilis CBS 621 can cope with hyperbaric stress, meaning that the use of increased air pressure is a suitable method for oxygenation enhancement of high-density cultures of this strain. (C) 2013 Elsevier Ltd. All rights reserved.