Frequency response properties of L-type S-potential in the retina in situ of the Japanese flounder Paralichths olivaceus were examined by flicker light stimulus. Three light intensities for the flicker were used. At the lowest intensity, response amplitude of the S-potential decreased monotonically as flicker frequency became higher. At the higher intensities, however, the response amplitude had a sub-peak around 20-30 Hz. We supposed that this phenomenon was some resonance caused by light adaptation of the retina as the light intensity of the flicker, therefore retinal mean illumination, increased. It is thought that S-potential can respond only to low frequencies. However, the obtained critical fusion frequency (CFF) in the experiments was 60 Hz or more, indicating that S-potential is able to respond over 60 Hz in the retina in situ of the Japanese flounder.