A series of fed-batch experiments at different agitation speeds were performed using the industrially important strain Trichoderma reesei RUT C-30 in two different bioreactors; to understand the close relationship that exists between the shear field within a bioreactor, the morphology of the microorganism, the rheology of cultivation broth, and the process performance. The two bioreactors, stirred tank bioreactor (STB) and reciprocating plate bioreactor (RPB), are characterized by a significantly different shear field to which microorganisms are exposed. Highest biomass concentration (ca. 15 g l(-1)) was obtained at higher agitation rates in both bioreactors due to better oxygen supply. However, better filter paper activities per mg of protein were obtained at lower agitation in both bioreactors. in both bioreactors, young and healthier fungi in the batch phase were not affected by shear even at higher agitation rates. However, during the fed-batch phase, higher degree of fragmentation of clump morphology at high agitation intensity was confirmed by image analysis. Also, the rheological analysis showed an increase in apparent viscosity during the batch phase and early fed-batch phase due to the increase in the biomass concentration. During the late stages of cultivation, the apparent viscosity decreased due to cell lysis and spore formation. (C) 2009 Elsevier Ltd. All rights reserved.