It seems historiographically implausible to ascribe the reputation of fin-de-siècle Lyon as France’s Bayreuth to the impact of a single middle-ranking soprano, but the Danish singer Louise Janssen’s long-term presence, galvanic musical influence, and box-office value suggest precisely that conclusion. Part of the explanation lies with the diva-worship of her supporters (‘Janssenistes’), who curated her image both during her career and in her retirement to create an adopted musical heroine whose memory remains guarded by Lyon council policy. That image, selectively constructed from among her Wagner roles, also typecast her as a singer who had much in common with Symbolist art – a potential Mélisande that Lyon never saw. This essay brings together archival and press materials to explain how a foreign-born singer’s agency and mythification contributed to a double French naturalisation – her own, and that of Wagner(ism).