Weight change in age-0 coho salmonOncorhynchus kisutchat about natural density was negatively related to the density of juvenile steelhead (anadromous rainbow troutO. mykiss) in a 6-week experiment conducted in July–August 1993 in the north and south forks of Caspar Creek, California. The experiment used 12 enclosed stream sections, each containing a pool and a portion of upstream riffle, with two replicates of three steelhead densities—zero, natural density (1×), and twice the natural density (2×)—on both the north and south forks. The natural density of coho salmon was about one-sixth the density of steelhead. Coho salmon survival was high (87% overall) and not related to treatments. In the north fork, coho salmon weight change was positive in zero density steelhead treatments, zero in 1× treatments, and negative in 2× treatments. Coho salmon weight change in the south fork was less favorable than in the north fork but was also negatively related to the density of steelhead. These results indicate that under some conditions resource partitioning by salmonid species does not eliminate negative interspecific interactions.