Formation of ocular cataracts after seawater (SW) exposure of yearling coho salmon has been studied in relation to the parr-smolt transformation. No ocular opacity was observed in fish in fresh water (FW). In February and March, survival rates 12 h after exposure to SW were 11 and 20%, respectively, and cataracts were observed in all survivors. Cataracts persisted even after prolonged adaptation to SW. In April, when yearling coho smolts migrate naturally to the sea, the incidence of cataracts decreased to 40% in the first week, none in the second and third weeks, and 14% in the last week of the month. Although no mortality in SW was seen from May to August, cataract incidence increased to 33% in May and 42% in June-August. The increase in cataract formation coincided with a decrease in SW adaptability. The time course of cataract appearance was studied in yearling masu salmon. In February, cataracts appeared 1 h after transfer to SW, and more than 80% of the fish showed cataracts after 3 h. When a lens of masu salmon was exposed to hypertonic salmon Ringer's solution, the transparent lens fibers became opaque within 1 h, indicating that cataracts may be induced by increased osmolality of the aqueous humor or by dehydration of the lens. Ocular opacity was limited to the area immediately below the lens epithelium around the suture line, where the tips of the cortical lens fibers meet at the center of the corneal (outer) side of the lens. The cataract seems to arise from structural damage to cortical fibers. The occurrence of cataracts after SW transfer may provide a simple and accurate method of evaluating the degree of smoltification of salmonids.