The behavior and stream habitat utilization of juvenile hatchery-reared coho salmon (Ovicozihynchus kisutch) and wild cutthroat trout (Salmo clarki) were examined to assess the nature of interactions between the species potentially resulting from introductions of hatchery coho salmon in streams. Habitat utilization was monitored in Mill Creek, Polk Co., Oregon by manipulating the composition of fish within three study sections to include either allopatric or sympatric populations of hatchery coho salmon and wild cutthroat trout. In allopatry, coho salmon and cutthroat trout selected feeding microhabitat that was similar in physical characteristics and exhibited extensive dietary overlap. When the two species occurred in sympatry, feeding microhabitat used by coho salmon was altered, however, that of cutthroat trout was comparable to allopatric cutthroat trout. No discernable differences in dietary composition between allopatric and sympatric fish were detected. While allopatric cutthroat trout always gained mass. ailopatric coho salmon always lost mass. Stomach fullness indices were higher for allopatric cutthroat trout than allopatric coho salmon. Sympatric cutthroat trout had comparable or greater gains of mass, indices of stomach fullness, and hepatic glycogen reserves relative to allopatric cutthroat trout. Conversely, sympatric coho salmon had greater losses of mass, reduced hepatic glycogen reserves, and apparently lower indices of stomach fullness than allopatric salmon. Behavioral observations made through the use of an in-stream viewing chamber in Berry Creek, Benton Co., Oregon, indicated that in allopatry populations of hatchery coho salmon and wild cutthroat trout were structured as linear dominance hierarchies that were organized by quantitatively and qualitatively similar aggressive interactions. In sympatry, however, cutthroat trout were more aggressive and socially dominant to coho salmon. The shift in use of micro-habitat and reduction in habitat utilization effectiveness of hatchery coho salmon in the presence of cutthroat trout can, thus, be interpreted to occur as a result of interference competition mediated through interspecific aggression.