Algae have been proposed as a source of biofuels and high value chemical products, but if this potential is to be fully realised, it is crucial to understand the factors a↵ecting the suspension rheology. Suspensions of three algae species, Tetraselmis chuii, Chlorella sp. and Phaeodactylum tricornutum, were sheared in a rotational rheometer in order to characterise their rheology and examine the e↵ects of cell concentration, motility and morphology. The volume fraction ranged from 0.05 to 0.2, and the shear rate from 20 to 200 s 1. The rheology measurements are fitted to the Herschel-Bulkley model, and the intrinsic viscosity is estimated using both Einstein's equation and the Krieger-Dougherty model, which are found to perform well for low concentrations. The intrinsic viscosity of T. chuii suspensions is shown not to be constant, but decreases with strain rate, indicating that the suspension viscosity is less sensitive to the cell concentration at high strain rates. The rate of decline is constant for strain rates below approximately 100 s 1 , after which it continues to decline linearly but at a slower rate. It is speculated that this transition at