In honour of Professor B. I. VERKIN'S 70th birthday High purity molybdenum single crystals (residual resistivity ratio x 6 . lo4) were studied by means of computer-controlled internal friction technique at frequencies of about 100 kHz in the temperature range 6 t o 300 K. The amplitude dependences of decrement were measured within the vibrational strain amplitude lo-' t o It was established that the temperature dependence of ultrasonic amplitude providing a constant level of reversible dislocation deformation coincides well in relative units with the temperature dependence of the critical resolved shear stress at 20 t o 180 K.