This the fourth report completed under terms of U.S. Government Contract for the experimental study of methods for the control of movements of downstream migrant salmon and steelhead trout. The experimental data were gathered between 12/17/53 and 6/8/54. The reactions to light of groups of hatchery raised and/or wild fish of all species of Pacific salmon and steelhead trout were studied. The range of sizes for each of the species were as follows: silver salmon (Oncorhynchus kisutch), 31-267 mm; chinook (O. tshawytscha), 35-168 mm; sockeye (O. Nerka), 26-185 mm; pink (O. gorbuscha), 31-39 mm; chum (O. keta), 34-45 mm; steelhead (Salmo gairdnerii), 28-275 mm. All experimental work was done at night in a darkened laboratory. Electric lamps were placed over and adjacent to the two ends of an aquarium with a glass front. Three minutes after a group of fish had been put in the aquarium, the lamp on the left was turned on for one minute, and the distribution of the fish in the upper versus the bottom half and left versus right half was determined. This was repeated four times for each group with the lighted lamp on the left, right, right, and left ends. One minute of darkness separated each light period. Next with both lamps on continuously, a piece of plywood was floated on the the water at alternate ends for four trials and the distribution of the fish in the light or shadow again recorded after each trial. Both lamps were then turned off and the number of fish entering the beam from a two-cell flashlight which was directed into one corner of the aquarium for one minute, was recorded. In general, all species of downstream migrant salmon tested showed a statistically significant preference for the darker portion of the light gradient. The sounding and light avoiding barrier displayed by all species of dark adapted young Pacific downstream migrant salmon and steelhead trout would seem to offer an effective, inexpensive and harmless method to assist in guiding those species in which the migration occurs primarily at night.