Submerged cultures of filamentous fungi are widely used in industry to produce a variety of economically important proteins, such as cellulase enzymes. One of the possible strategies to increase enzyme yields relies on enhancing biomass concentration before production. However, when biomass concentration is increased, the culture broth becomes highly shear-thinning, affecting both mixing and oxygen transfer. Fungi and enzyme yields depend strongly on the ability to supply oxygen to the broth and, in consequence, scale-up considerations mainly focus on gas-liquid mass transfer predictions. In this study, rheology and mass transfer are characterised during the growth of Trichoderma reesei. A power law model is proposed to simulate the rheology of the broth, based on a consistency index that depends on the concentration of the biomass. Mass transfer coefficients measured during the growth phase in bioreactors of 3 and 20 L are compared to those measured with water and model fluids. As the value of power consumption is important for the mass transfer correlations, special care was devoted to validating its measurement in the larger vessel. Comparisons between experiments and recently developed correlations showed that the use of model fluids is a good way to investigate hydrodynamics and mass transfer inside large mock-ups. Good agreement was observed between mass transfer coefficients measured during the growth phase and those calculated using a correlation developed from model fluid experiments (Gabelle et al., 2011). The tools established here to estimate rheology and mass transfer during the T. reesei growth phase could be used in the future for cost estimation and process optimisation. (C) 2012 Elsevier Ltd. All rights reserved.