Ionic conductivity in the extrinsic range of temperature (250° to 400°C) was measured for specimens taken from a single‐crystalline ingot of KCl subjected to zone purification. The effect of dislocation density was studied in order to clarify the nature of ionic conduction in these highly purified crystals. Conductivity increased by a factor of 10 in the purest crystals by increasing dislocation density from 106to 107cm−2. Conductivity was decreased in the impurer crystals by increasing the density of dislocations. Comparing the results with predictions from charged dislocation theory, the value of 0.7 ev is obtained for the formation energy of positive ion vacancies.