This histological study is concerned with demonstrating the existence of retinomotor changes in flatfishes. In three species, selected as representatives (sole, Solea solea; plaice, Pleuronectes platessa; and merry sole, Microstomus kitt), it is shown that normal radial displacements of rods, cones, and retinal pigment take place in expected directions in light and darkness, with exceptions which may be important but whose implications have still to be revealed. In the plaice the cones of the dorsal retina are permanently retracted against the basement membrane, and mobile elsewhere. The eyes of flatfishes, therefore, conform to those of other teleosts in photomechan-ical responses to illumination.