The ultrasonic attenuation due to dislocations was studied in the range 3.6-30 MHz in a single crystal of tin doped with 0.0016 at% indium at 1.3K in the normal and superconducting states. The attenuation at various pulse amplitudes shows peaks in both states. The attenuation peaks due to the breakaway of the dislocations from minor pinning points were used to calculate the dislocation resonance frequency. The breakaway stress is found to decrease with increasing frequency in both states. The attenuation shows resonance-type behaviour as a function of frequency. In the superconducting state unpinning starts at a lower pulse amplitude and there is a systematic decrease of the resonance frequency with increasing amplitude. In the normal state there is evidence of unpinning at higher values of pulse amplitude. The nature of the variation of attenuation with amplitude and frequency in both the states has been explained on the basis of Rogers theory (1962).