1. A method has been devised for measuring the electrical conductivity of single crystals at elevated temperatures and at field strengths below 3 volts per mm. 2. The conductivities of single crystals of quartz, periclase, and corundum have been measured by this method over a temperature range of 600° to 1400°C, in equilibrium with air. 3. The increase of conductivity with temperature follows a single exponential relation in all cases, throughout the temperature range covered. Experimental curves and a table of interpolated values are given.