All commented on Themistocles’ feat of mastering the Persian language. I show how Plutarch’s depiction of Themistocles differs from previous accounts in cultural assimilation and language acquisition. I argue that Plutarch has been influenced by contemporary concerns relating to assimilation and ethnicity. I analyze three incidents: the inscription Themistocles allegedly inscribed on the shores of Euboea after Artemisium (Them. 9.2), the report that Themistocles ordered that an interpreter should be killed “on the grounds that he dared to use the Greek language in the service of barbarian overlords” (Them. 6.1), and his studies of the Persian language (Them. 29). Plutarch either includes events found in no previous source, or presents them in a tendentious manner different from previous accounts. For Plutarch, the Themistocles legend had become a battle ground for questions of language and self-definition. This trend is continued by authors after Plutarch, who emphasize and invent incidents in Themistocles’ life which demonstrate the primacy of the Hellenic language and culture.