Among the three species of Pacific salmon Oncorhynchus established in the Laurentian Great Lakes, juveniles of coho salmon O. kisutch are most likely to compete with brook trout Salvelinus fontinalis and brown trout Salmo trutta for food and space, because their juvenile life histories and ecologies are similar where they rear together in tributary streams. Coho salmon emerged 2–3 weeks earlier and were 6–8 mm longer than brook or brown trout at emergence during 1979 in Lake Michigan tributaries where the species were sympatric. During the first summer of life, coho salmon generally were 6–21 mm (7–54%) longer and weighed 0.3–4.1 g more than brook and brown trout. Size at emergence and first-year growth were similar among brook trout and brown trout. In laboratory stream experiments with pairs of the three species, coho salmon dominated brook or brown trout of equal size, and brook trout dominated equal-size brown trout. Competitive superiority of dominant species was based on the ability of fish to defend energetically profitable stream positions during sympatry. When released from competition, subordinate species shifted to use more profitable positions. Specific growth rates of coho salmon in the laboratory equalled rates measured in Lake Michigan tributaries. However, brook and brown trout grew more slowly in the laboratory than in the field, probably because intraspecific competition was high due to lack of cover affording visual isolation. Results suggest that the larger size and competitive superiority of coho salmon should give them an advantage over juvenile brook and brown trout in Great Lakes tributaries when resources become limiting.