The blood of two Australian marsupials, the eastern grey kangaroo (Macropus giganteus) and the Tasmanian devil (Sarcophilus harrisii), has been reported to have greater oxygen-carrying capacity (i.e. haemoglobin content) when compared with that of placental mammals. We investigated whether alterations of blood rheological properties are associated with the increased oxygen-carrying capacity of these marsupials. Eastern grey kangaroos (n≤6) and Tasmanian devils (n≤4) were anaesthetised for blood sampling; human blood (n≤6) was also sampled for comparison. Laboratory measurements included blood and plasma viscosity, red blood cell (RBC) deformability, RBC aggregation and the intrinsic tendency of RBC to aggregate, RBC surface charge and haematological parameters. Scanning electron micrographs of RBC from each species provided morphological information. High-shear blood viscosity at native haematocrit was highest for the Tasmanian devil. When haematocrit was adjusted to 0.4LL -1, lower-shear blood viscosity was highest for the eastern grey kangaroo. RBC deformability was greatly reduced for the Tasmanian devil. Eastern grey kangaroo blood had the highest RBC aggregation, whereas Tasmanian devil RBC did not aggregate. The surface charge of RBC for marsupials was ∼15% lower than that of humans. The dependence of oxygen-delivery effectiveness on haemoglobin concentration (i.e. oxygen content) and blood viscosity was quantitated by calculating the haematocrit to blood viscosity ratio and was 1525% lower for marsupials compared with humans. Our results suggest that environmental pressures since the marsupialmonotreme divergence have influenced the development of vastly different strategies to maintain a match between oxygen demand and delivery.