The gas‐breakdown threshold or the minimum power density required to ionize a gas with 10.6‐&mgr;‐wave‐length radiation has been examined using the radiation of aQ‐switched CO2laser. The studies show that the focused beam cannot initiate the breakdown process for intensities as high as 109W/cm2. If an initial low degree of ionization is provided by an external source, the subsequent growth of the breakdown is in agreement with a cascade model as evidenced by the experimentally determined gas pressure and laser‐radiation frequency dependence.