A new type of discharge instability is reported for a low‐pressure Na–noble‐gas discharge (∼5×10−3Torr Na; 5.5 Torr Ne‐Ar, 1% Ar). The voltage‐current (V‐I) characteristic is shown to be multivalued in the voltage. The resulting instability is a discontinuity in the positive column which propagates in the direction of electron particle flow with velocity between about 105and 5×105cm/s.