Abstract: Two captive bottlenose dolphins housed in a float cage in a Bay were subjected to a continuous 20 kHz tonal signal.The behavioral responses of dolphins were judged by comparing the surface distances relative to the sound source,number of surfacings,and number of the echolocation clicks production of the dolphins,during test periods and baseline periods.The avoidance threshold sound pressure level (SPL) for a continuous 20 kHz tonal signal for the bottlenose dolphins was estimated to be 154±2 dB(re 1μPa,rms).In addation,the comparison between the avoidance threshold SPL for harbor porpoises to a 50 kHz continuous signal and bottlenose dolphins to a 20 kHz continuous signal was also studied.The results showed that the dolphins moved away from the sound source and came to the surface more frequently during the test periods than the baseline periods,but they produced clicks significantly fewer.Therefore,a continuous 20 kHz tonal signal seems to deter bottlenose dolphins from an area.