A 64-foot approach to the University of Washington fishladder was wired so that electric lights underneath the screen could be turned on and off on alternate nights during the adult salmon runs. The number of adult sockeye, chinook and silver salmon ascending the ladder was recorded daily. All species displayed a statistically significant preference for entering during the night over the day, and for dark nights over light nights. Sockeye were significantly more sensitive to light than either silvers or chinooks, with no significant difference between the latter two species.