The adiabatic elastic constants of holmium were determined in the temperature range from 78 to 300 K, using high‐purity single crystals. The constants were obtained from the ultrasonic‐wave velocities (20 MHz) measured by the pulse‐superposition method. The values of the constants at room temperature in units of 1011dyn/cm2arec33=8.015,c44=2.592,c11=7.612,c12=2.600, andc13=2.072. Using these values, the Debye temperature was computed to be 181 K. The sound velocities tend to vary with temperature in an anomalous fashion near the Ne´el temperature(TN=131 K). Attenuation and sound‐velocity measurements nearTNpermit an estimate of the relaxation time for spin fluctuations, which was found to be 3×10−10sec.