A very intense gas breakdown spark has been extended to a continuous arc in argon using a focused cw CO2laser. To achieve cw breakdown, the focal volume of a mirror focusing a high‐power CO2laser was preionized by a single pulse from a CO2TEA laser. The electron density created by the pulsed laser is sufficient to start the cw plasma. This letter reports accurate measurements of pulsed thresholds as well as preionized cw thresholds for breakdown in argon. Also, a study of the time development of the cw plasma is presented.