The thermal contraction of ice single crystals between 0 and −50 °C has been measured by the use of silicon strain gauges frozen on the crystal basal surface. The experimental results cannot be explained without assuming the existence of a Newtonian viscous layer at the ice-strain-gauge interface; this layer has a thickness which changes in temperature as predicted by Fletcher, and a viscosity intermediate between that of ice and that of supercooled water. The activation energy of the viscosity, (0.36±0.04) eV, seems to indicate that it is controlled by a vacancy mechanism in which only the vacancy migration is effective.