Using a classical cardiac conditioning technique, the auditory thresholds of marbled sole Pleuronectes yokohamae were measured in the presence of masking noise in the spectrum level range of 70-90dB re 1μPa/√Hz. Critical ratios (signal-to-noise ratio) were approximately 27dB at 60-100Hz and 32-58dB at 150-700Hz. The ratio increased almost linearly with increasing frequency. The noise spectrum level at the start of masking was 62-70dB within the frequency range of 60-700Hz. This suggests that hearing of the marbled sole is masked in the natural environment with the noise spectrum level above 70dB.