Microspectrophotometric analysis of the visual receptors of "yearling" brown trout, Salmo trutta, revealed three cone types, double cones with visual pigments absorbing maximally at about 600 and 535 nm, and two types of single cone with lambda max at about 440 and 355 nm. Two-year-old fish did not possess the u.v. cone cells. Microscopical analysis of the cone mosaic in "yearling" trout showed a square pattern of double cones with a central single cone and corner single cones, but in two-year-old trout the corner cones were absent. It is concluded that u.v. sensitivity is derived from the corner cones of the mosaic, and that it is only present in young trout.