A laboratory-scale microbubble dispersion (MBD) generator was shown to improve oxygen transfer to aerobic microorganisms when connected to a conventional air sparger. However, this process setup had not been tested with fungi, where the morphology of microorganisms causes a very viscous broth rheology that impedes the oxygen mass transfer. Trichoderma reesei RUT C30 was chosen as a model fungus as it shows both the challenging rheology and a cellulase enzyme complex product that is shear-sensitive. Conventional and MBD sparged fermentations where carried out to compare the effect of aeration methods on oxygen transfer and product formation. MBD sparged fermentations showed fivefold higher k L a values and nearly twofold higher cell mass productivities. The cellulase enzyme activities were similar for both the MBD and the conventional sparged fermenters.