Subyearling coho salmonOncorhynchus kisutchand steelheadO. mykisswere placed together and separately in laboratory stream channels to assess their habitat use, interactions, and response to cover and predators. We studied the fish in six replicated riffle-pool-riffle units in the laboratory channels during 12 trials of 7 d each. In all test situations, we allowed the fish to emigrate from the test units. More fish remained in the units of the laboratory streams when both species were together than when either species was alone. The presence of larger fish (potential predators) caused both species to change habitats, regardless of cover available, and many left the test units or were eaten. Coho salmon used mainly the pools, whether stocked alone or together with steelhead; steelhead used both the riffles and pools when alone but shifted slightly toward increased use of the riffles when both species were present.