The room-temperature attenuation of longitudinal acoustic waves propagating in the c direction in single crystal beryllium oxide (BeO) has been measured from 0.3 to 1.2 GHz. The attenuation follows a frequency squared dependence with a measured attenuation of 1.1 dB/cm at 1.0 GHz. The magnitude and frequency dependence of the loss indicate an intrinsic interaction with thermally excited phonons characteristic of the ωτ ≪ 1 region. The loss level compares favorably with a value of 1.4 dB/cm predicted from thermal conductivity and crystal structure considerations [D. W. Oliver and G. A. Slack, J. Appl. Phys. 37, 1542–1548 (1966)]. Using the WE lattice loss theory [T. O. Woodruff and H. Ehrenreich, Phys. Rev. 123, 1553–1559 (1961)] a Grunheisen γ of 0.54 is calculated. This compares to longitudinal mode c-axis Grunheisen constants of 0.78 for zinc oxide and 1.51 for cadmium sulfide, binary compounds of the same crystal class. Empirical relationships were developed between crystal constants and ultrasonic attenuation for the II–VI compound hexagonal crystal class.