At the end of Book 15 of his Res Gestae Ammianus Marcellinus reports how Strategius Musonianus became the successor of the murdered Domitianus as Praefectus Praetorio Orientis (PPO). He tells that Strategius was a man versed in the two languages, i.e. Greek and Latin, and that because of this he had won a higher distinction than was expected. When Constantine the Great, so says Ammianus, was looking for an expert interpreter for his investigation into the Manichaean and similar heresies, he chose Strategius as a person recommended to him as competent. Constantine was so pleased with Strategius' skilful work that he gave him the honourable nickname Musonianus, which became his official name. This assignment was the start of a great career which eventually led to his denomination as PPO under Constantius II.