The increase in the angling for steelhead trout (Salmo gairdneri irideus) since the end of World War II caused fishery biologists in the State of Washington to re-evaluate their management policies and to search for a means of building up runs of adult steelhead trout in depleted or heavily fished streams. Experiments were conducted to determine the proper size and time of planting of hatchery-reared steelhead trout to obtain best returns of adults. It was found that the planting of migrant-sized steelhead trout during the normal migration period of wild fish provided the best returns. Runs of adult steelhead trout have been built substantially up by plants of hatchery-reared fish as evidenced by trap returns on planted streams.