Frederick II (1712-1786) preferred speaking French to German. Alongside the educational and culture-historical reasons for this preference, this philological study systematically examines the Prussian king's bilingualism. His usage of French and German is subjected to a linguistic analysis with reference to an extensive corpus of manuscripts. The detailed description of the monarch's use of these two languages in his everyday concerns as ruler, military commander, private individual, writer, and reader provides a methodologically novel portrait of the bilingual roi-philosophe.