The frequency dependence of microwave elastic losses for longitudinal waves along thecaxis of a &bgr;‐rhombohedral boron crystal have been measured from 0.5 to 1.8 GHz at 4 and 300°K. At 1 and 1.5 GHz, the temperature dependence of these losses has been determined from 4 to 300°K. At 300°K, the attenuation due to interaction with thermal phonons is found to vary quadratically with frequency, in accord with theory, and to have a value of 0.1 dB/cm at 1 GHz. The sound velocity of the longitudinal wave along thecaxis is found to be 1.4 × 106cm/sec.