This article interrogates cosmopolitanism in Henry James’s ‘Daisy Miller’, arguing that transatlantic mobility and travel expose America’s residual postcolonial insecurity. Fear of transatlantic acculturation undermines the national ideology and identity of the American Adam, as incorruptible in his fundamental innocence. By tracing the language of contagion surrounding biological pollution, this analysis examines how anxieties surrounding transatlantic flirtation, acculturation, and sexual union, in James’s text, expose America’s post-revolutionary fear of cultural permeability and fragility. When cosmopolitanism reveals American culture to be porous, this threatens its ideological self-definition, attesting to the imaginary and mythologized nature of the founding Adamic belief. I argue that ‘Daisy Miller’, focuses on the question of what happens to Americans when they have lived too long in Europe and how acculturation affects self-knowledge. Told from the perspective of Europeanized Americans, these American expatriates in the cosmopolitan residences of Geneva and Rome appear unaware of the extent of their acculturation until the naïve all-American girl Daisy, through her unfamiliar and highly ambiguous manners of flirting, appears to disrupt codes of female propriety and the Europeanized Americans’ perceived nativist loyalty. The novel acts as provocations to American characters who have very fixed ideas about what national identity is, and this article will trace how transatlantic flirtation and the subsequent relationships it produces, become a disruptive force. This article will show how fear of cultural flirtations derives from fear of potential sexual contact and thus interbreeding.