We use the case study of coho salmon and steelhead trout to discuss the coexistence of two similar stream salmonid species. The question of coexistence between these two species dates back to the early 1960s, making it a good example of the application of ecological theory to explain patterns observed in nature. Explanations for habitat selection and segregation between coho salmon and steelhead trout have included the mechanisms proposed by Nilsson (The biological basis of freshwater fish production. John Wiley and Sons, New York, 1967), interactive segregation and selective segregation. Through examples of laboratory and field studies of coho salmon and steelhead trout in Southeast Alaska, we suggest that habitat selection appears to be largely selective, but interactive segregation may become important during times of resource limitation. Despite a half-century of scientific inquiry into the coho-steelhead coexistence question, there is still much to be learned about habitat selection of these species; given the precarious status of many populations of coho salmon and steelhead trout within their native range, efforts to understand their ecological requirements are warranted.