A bite from the Aedes aegypti mosquito can transmit pathogenic viruses such as dengue, Zika, and chikungunya. While public health campaigns have historically focused on eliminating this vector species, a project in Rio de Janeiro, Brazil, proposes to release A. aegypti carrying the intracellular bacterium Wolbachia. This microbe, passed from female A. aegypti to their progeny, would hinder viral transmission, making the bite less of a threat and promoting a more convivial coexistence. In releasing Wolbachia mosquitoes, project proponents presented their strategy as a solution for disease control that harnessed “nature,” instead of working against it. By juxtaposing historical accounts and ethnographic research in Rio with descriptions of biological processes, this article investigates how the microbe-mosquito dyad was turned into a biotechnology of epidemic control. Proponents of the Wolbachia project hoped that a change within multispecies relations—the novel human-bacterium-mosquito relationship—would change other multispecies relations—the historically constituted human-virus-mosquito relationships in Rio and beyond. This shift, which I term “multispecies transmutation,” highlights how relational alterations in both biologies and socialities created a new life-form (“Wolbachia mosquitoes”) and a new form of life (of multispecies coexistence). By tracing the more-than-human biopolitical arrangements put forward by the project, this article explores the recalibration of technoscientific solutions to global environmental health concerns. Multispecies transmutation offers a new framework for understanding the management of multispecies relations, showing how techniques of governance can target not individuals or populations per se, but rather interspecies connections.