Retinal photoreceptors of six-month-old Pacific salmon fry (Oncorhynchus masou) (masu salmon) were found to respond to the ultraviolet radiation (UVR) in different combinations with constant magnetic field (MF) (80 G) in a special, previously undescribed manner. After exposure to UVR for 2 hours (1st experiment) the pigment index (PI) of retinomotor response increased from 0.30 +/- 0.10 to 1.31 +/- 0.20. The rods were located within the pigment layer, while cones, including double ones, were only partially screened with pigment. The whole retina demonstrated twilight (mesopic) response. After exposure to UVR for 2 hours and subsequent 2 hours-long exposure to MF (2nd experiment), PI rose to 2.08 +/- 0.10, while an average myoid height, and an average double cone diameter were increased. The whole retina demonstrated photopic response. In the 3rd experiment (2 hours of MF exposure, followed by 2 hours of a UVR exposure) value was minimal--1.28 +/- 0.12. Average myoid height was decreased, while an average cone diameter was unchanged. The whole retina response was scotopic (as under conditions of darkness). On the basis of authors' own results and the data from the literature, the conclusion is drawn that MF distorts influence of UVR, resulting in the unusual responses of the retina. The results obtained confirm the previously proposed variant of light-dependent magnetoreception model in the vertebrates.