Auditory frequency discrimination of marbled sole Pleuronectes yokohamae was determined by measuring the cardiac response of fish exposed to a series of paired tones, one of standard frequency (F1) and one of alternating-tone frequency (F2) were combined at 0.2 s intervals. Fish were first conditioned in a paired tone of different frequency in one octave coupled with an electric shock. Fish could discriminate frequencies between 100 and 300 Hz, but not frequencies of 400 Hz and above. Frequency discrimination limens ((|F2−F1|/F1)×100(%)) were approximately 30% at 100-200 Hz and 35% at 300 Hz in standard frequency.