A Cˇerenkov counter is described for detecting and measuring the energy of x‐rays in the 50‐ to 300‐Mev range. The incident x‐ray produces a photon‐electron shower in a cylinder of carbon tetrachloride, and the Cˇerenkov radiation emitted by the electrons in the shower is a measure of the x‐ray energy. The cylinder of carbon tetrachloride is 12 in. diam by 36 in. long. The Cˇerenkov radiation is measured with seven 5‐in. photomultiplier tubes mounted at one end of the cylinder. The pulse distribution from the counter has a &sgr;/x¯which varies from 0.146 at 217 Mev to 0.216 at 47 Mev. The energy response of the counter is linear up to 217 Mev, the highest energy at which measurements have been made.