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The retinomotor response of the retina was studied in Pacific Ocean masu salmon Oncorhynchus masou in conditions of mesopic (crepuscular) illumination with experimental compensation of the geomagnetic field (GMF) using a Helmholtz coil. In controls, the retinomotor response of masu fry to crepuscular illumination was normal: the nuclei of neurosensory rod cells were positioned immediately beneath the external limiting membrane and the nuclei of neurosensory cone cells were located closer to the pigment epithelium layer. In conditions of experimental compensation of the GMF, the retinal response of masu fry was altered: the nuclei of neurosensory cone cells were adjacent to the external limiting membrane, while rod nuclei were close to the pigment epithelium layer. Double and central neurosensory cone cells occupied a position inappropriate to the normal response to twilight: the bodies of these cells were significantly lengthened and their outer segments reached the pigmented epithelium layer. Thus, experiments using compensation of the GMF identified an unusual structure for the retina, which only partially corresponded to the response to mesopic adaptation. These data led to the conclusion that visible light is not the only variety of electromagnetic field perceptible to the retina in fish.