Magnetic susceptibility, transport and heat capacity measurements for single-crystalNaxCoO2 (x = 0.71) are reported. A transition to a spin density wave (SDW) state atTmag = 22 K is observable in allmeasurements, except χac data in which a cusp is observed at 4 K and attributed to a low temperature glassy phase.M(H) loopsare hysteretic below 15 K. Both the SDW transition and low temperature hysteresis are only visible alongthe c-axis. The system also exhibits a substantial positive magnetoresistance below this temperature. Calculations of the electronic heat capacityγ above and belowTmag and the sizeof the jump in C indicate that the onset of the SDW brings about the opening of a gap and the removal of partof the Fermi surface. Reduced in-plane electron–electron scattering counteracts the loss ofcarriers below the transition and as a result we see a net reduction in resistivity belowTmag. Sodium ordering transitions at higher temperatures are observable as peaks in the heatcapacity with a corresponding increase in resistivity.