Sixteen groups of 25 yearling salver salmon (Oncorhynchus kisutch) were tested for guidance in an experimental flume with 2 fps velocity under four combinations of light adaptation and barrier illumination. Five minutes of adaptation to an 8 ft. cdl. level significantly increased the guiding effectiveness of the 8 ft. cdl. light barrier and was accompanied by the longest swimming times in the flume. Adaptation to 100 ft. cdls. destroyed the guiding effectiveness of the 8 ft. cdl. light barrier and greatly reduced the swimming time. Any light in the flume significantly increased the swimming time over that obtained in the dark.