During the late 1930s, Great Lakes fishermen became concerned because of the new occurrence of the sea lamprey (Petromyzon marinus). Originally an Atlantic coastal fish, it was allowed to migrate throughout the Great Lakes due to various canal extensions. By drawing from literature on the sociology of environmental problems and animal invasions, this article traces how the sea lamprey became problematized as a threatening invader between the late 1930s and early 1970s. Throughout this period, a broad coalition of fishery biologists, fishermen, politicians, and journalists were involved in framing the problem. Although sea lamprey research, localized control practices, and environmental discourses considerably changed, the sea lamprey continued to be regarded as an invasive fish that was not allowed to exist in the Great Lakes. The case shows how these shifting ways of understanding the problem in fact led to the continuation of past management directions.