The longitudinal ultrasonic attenuation and velocity of L-arginine phosphate monohydrate (LAP) single crystal were investigated at 11.6, 15, and 30 MHz from 160 to 300 K during heating. An internal friction peak was found in all experiments. The frequency dependence of this peak indicates that it is associated with dislocation resonance damping. Although it is generally believed that the damping parameter B in the vibrating-string model of dislocation damping is approximately proportional to the temperature in metals, our results indicate that in LAP single crystal, parameter B can be expressed as B proportional to tau = me(-n/T), where tau is the relaxation time, T is the temperature, and m and n are positive constants.