The study of bi- and multilingualism in the ancient Mediterranean has come into its own in recent decades. The evidence is far greater for the Hellenistic and Roman periods than the Classical, so naturally scholarly attention has focussed less on the earlier era. This has led to some enduring notions about bilingualism in the fifth century b.c.e. which are yet to be fully scrutinized, including the idea that a Greek's speaking another tongue was inherently transgressive. What did it mean for a Greek to speak a second language? This article re-evaluates the evidence for individual bilingualism in Herodotus and Thucydides in their fifth-century context, focussed on our two best-documented examples of bilingual Greek individuals (Histiaeus of Miletus and Themistocles of Athens). Close reading of Herodotus and Thucydides suggests that not only does the notion of an inherently transgressive bilingualism hold little water for this period, but bilingualism may even be a sign of μῆτις.