Suspension rheology of fungal fermentations is important in determination of mass transfer rates, as well as mixing quality. We have characterizedTrichorderma reesei RUT-C30 suspension rheology during growth on xylose (soluble) and cellulose (particulate) substrates, using both steady and dynamic shear measurements. Biomass growth was monophasic on xylose and biphasic on cellulose; the latter behavior is consistent with relatively rapid, early growth on soluble sugars derived from rapidly hydrolyzed material, followed by a second, slower growth phase owing to hydrolysis of more recalcitrant cellulose by increasing cellulase concentrations.