Abstract.In the wake of the Franco-Prussian war, scientific nationalism became a subject of scientific controversy in Germany. This paper explores the controversy between the cosmopolitan physiologists Hermann von Helmholtz and Emil du Bois-Reymond on the one hand, and the nationalistic economist-philosopher Eugen Dühring and the astrophysicist Johann Carl Friedrich Zöllner on the other. It argues that Helmholtz’ frequent visits to Britain helped him keep abreast of scientific developments there and shaped his ideas of science and society. They also changed his (...) conception of the conservation of energy. German nationalists objected to his British contacts with offensive accusations. They alleged that he had plagiarized his fellow-German Julius Robert Mayer and tried to imitate British physicists. Thus Dühring and Zöllner defended and constructed enduring national stereotypes of German and British science. Whereas Helmholtz put scientific cosmopolitanism into practice, Du Bois-Reymond defended it as an ideology. In contrast to the received view of Du Bois-Reymond as a fervent nationalist, it is argued that he was actually an untimely scientific cosmopolitan. He placed himself in the tradition of Goethe and Schiller's Weimar cosmopolitanism. (shrink)