GALLO, COSTANZO (Flavius Iulius Constantinus Gallus). - Caesar from 351 to 354, son of Giulio Costanzo, half-brother of Costantino Magno, and of Galla, born at Massa Marittima in 325-26, died near Pola in December 354. Having escaped the massacres of 337 after the death of Costantino, he was taken, together with his brother Giuliano, to Nicomedia and entrusted to the Arian bishop Eusebio. In 343-46 he was transferred to Macellum near Cesarea di Cappadocia.
Suddenly, in 351, Costanzo II created him Caesar and charged him with defending the eastern frontiers of the Empire against Persian incursions. He then married the Emperor’s sister, Costantina, and established his residence at Antiochia di Siria. In government he proved cruel, punishing entire cities with bloody massacres and destruction in order to suppress their insurrections; and while he lived in dissipation and revelry, the true interests of the Empire were neglected. With his connivance, the praetorian prefect Domiziano and the quaestor Manzio were murdered by the people; Costanzo had placed them beside him to assist and supervise him in government. Costanzo then summoned him to Milano on the pretext of an exchange of views; but, having arrived in Istria, G. was arrested, summarily tried, and executed in the city of Flanona near Pola.