Rheological properties of microalgae slurries were measured as a function of biomass concentration from 0.5 to 80 kg/m(3) for Nannochloris sp., Chlorella vulgaris, and Phaeodactylum tricornutum. At biomass concentrations smaller than 20 kg/m(3), all slurries displayed a Newtonian fluid behavior with less than 30% increase in the effective viscosity from that of the nutrient medium. However, at biomass concentrations larger than 60 kg/m(3), the slurries of the green algae, Nannochloris sp. and C. vulgaris, displayed a shear thinning non-Newtonian behavior with varying degrees of sensitivity to shear rate while that of the diatom, P. tricornutum, was still a Newtonian fluid up to 80 kg/m(3). Moreover, bioenergy pumping effectiveness showed significant deviation among different species in the non-Newtonian regime. Finally, dewatering the slurries to concentration factors larger than 80 did not further increase the total bioenergy harvest effectiveness. (C) 2011 Elsevier Ltd. All rights reserved.