The operation of a new type CO chemical laser has been demonstrated in which a chain reaction is initiated by ac discharge through O2discharge tubes and the laser cavity. The chain reaction with CS/O2as the fuel/oxidizer,and O and S atoms as the chain carriers, is thought to be the basis of the operation. Gain and power measurements have been made and small‐signal gains of 1.1%/cm for bothP8(18) andP6(17) have been observed 7.5 mm downstream from the CS2transverse injectors. Laser oscillation has been observed directly below the CS2transverse injectors.