Endangered species are objects of intense scientific scrutiny and political conflict. This article focuses on the interplay among human-nonhuman relations, knowledge production, and the politics of endangerment. Advancing a historical ontology of endangerment, it highlights the role of transforming the nonhuman world in the coming to be of new objects of environmental knowledge. Such knowledge can provide the basis for credible claims of endangerment, facilitating mobilizations against the very human-nonhuman relations that produced it. An in-depth case study of the delta (...) smelt, an endangered species of fish caught in the center of California’s “water wars,” shows how changes in the instrumentalization of the nonhuman environment can produce new knowledge of nature that allows actors to make claims and form coalitions that would be otherwise inconceivable. Because its sole habitat is the hub of California’s water delivery system, efforts to save the species from extinction have reduced flows to farms and cities, fomenting conflict between environmentalists and water users. This article demonstrates that the taxonomic classification of the delta smelt as a species and evidence of its decline arose directly from the reengineering of California’s rivers for extractive ends. Ironically, the knowledge on which environmental advocates relied was a product of the instrumental relation to nature that they sought to transform. (shrink)