The density of both H$_2$0 and D$_2$0 vapour has been measured to a precision of 0.1-0.3 mmol dm$^-3$ or 2-6 $\mu$g cm$^-3$ in the range 150-500$^\circ$C by injecting known volumes of liquid water into a vessel of known volume. The maximum pressure was kept low enough at each temperature that neither adsorption nor capillary condensation in the screw threads of the pressure vessel contributed significantly to thc measurements. The densities were analysed to determine the second and third virial coefficient by least squares. A more accurate estimate of the second virial coefficient was obtained by accepting thc value of the third virial coefficient so obtained and extrapolating thc apparent second virial coefficient, as obtained from the measured densities and the values of the third coefficient, to zero pressure. The second virial coefficient of D$_2$0 vapour is significantly less than that of H$_2$0 vapour, no doubt because of the differences of the nuclear zero-point and thermal energies.