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 kLa values and nearly twofold higher cell mass productivities. The cellulase enzyme activities were similar for both the MBD and the conventional sparged fermenters.