Reactions of Japanese anchovies Engraulis japonicus to 100 to 700 Hz underwater pure tone signals were observed. The mean body length of the anchovies was 11 cm and mean weight was 10 g. Seven hundred fish were kept in a net enclosure which was 2 m in diameter and 1.5 m in depth. The anchovies ordinarily swam in a circle at almost same speed in the net enclosure. The startle response behaviour was defined as follows: more than a half of the fish changed their behaviour just after the sound projection, for example, the acceleration of the swimming speed, dispersion or concentration of the fish group. When no or few anchovies changed their ordinary behaviour, it was defined as no response. The up/down staircase method was used to determine the threshold level of the startle response. The sound pressure level ranged from 130 to 160 dB and was changed by 5 dB steps. The startle response levels were 154.5 dB at 100 Hz, 153.3 dB at 200 Hz, 146.8 dB at 300 Hz and 153.8 dB at 500 Hz. The 700 Hz signal did not affect the fish behaviour up to 158 dB.