During the months of August and September 1957, a series of experiments were conducted at Bonneville Dam, to determine (1) how adult migrating salmonids respond to differences in flow velocity, (2) how they perform in two relatively high-velocity flows, and (3) how the velocity of flow influences their rate of movement. Given a choice of entering either of two parallel channels carrying flows of different velocities, steelhead trout (Salmo gairdneri), chinook salmon (Oncorhynchus tshawytscha), and silver salmon (O. kisutch) generally demonstrated a preference for the channel with the higher velocity flow. The magnitude of the response varied between species and with velocities of the choice condition. The performances of steelhead trout and chinook salmon were examined in flow velocities of 13.4 and 15.8 feet per second by determining the distance they could achieve in an 85-foot channel. Although there was considerable variation in the distances attained by individual fish at each velocity, steelhead trout were generally more successful in negotiating these velocities than chinook salmon. Larger fish of both species were more successful in negotiating the two flows than smaller fish. Both species performed better in the 13.4 feet per second flow than in the 15.8 f.p.s. flow. Rates of movement of steelhead trout, chinook salmon,and silver salmon were measured in velocities ranging from 2 to 15.8 f.p.s. Rates of movement varied with species, size, of fish, and velocity. Maximum observed swimming speeds are given for each species and various factors affecting rate of movement are discussed.