Abstract: The electroretinogram(ERG)of intact eyes were recorded to study the retinalphoto-sensitivity and spectral sensitivity of a nocturnal predator, the mandarin fish,Siniperca chuatsi. The ERG forms of the fish are almost the same at scotopia and photopiaexcept that a small d-wave was observed at photopia when evoked by the strongest stimuluslight. The threshold value in dark-adapted state is 1,000 times smaller than that inlight-adapted state.The scotopic and photopic spectral sensitivity curves are identical in shape, with theirpeaks being located at a wavelength near 530nm. The Purkinje phenomenon was not observed in this fish.At both weak and intense background illuminations, only can the decreased type oflight-adaptation curves be seen in the change of b-wave sensitivity.When retina is previously light-adapted by an intense background illumination,the threshold changes slowly andcannot restore the threshold of fully dark-adapted retina after three hours.There are noshoulders or turnings recorded in dark-adaptation curves of the fish.Since only one receptor system is present in the retina of the mandarin fish,we can thussuggest that this fish is of colour blindness,and it improves its photo-sensitivity by theabandonment of colour vision,so as to enable itself to recognize preys in a very dim light.