The Diocese of Egypt was a diocese of the later Roman Empire, incorporating the provinces of Egypt and Cyrenaica. Its capital was at Alexandria, and its governor had the unique title of praefectus augustalis instead of the ordinary vicarius. The diocese was initially part of the Diocese of the East, but in ca. 380, it became a separate entity, which lasted until its territories were finally overrun by the Muslim conquest of Egypt in the 640s.
Administrative history
Egypt was formed into a separate diocese in about 381. According to the Notitia Dignitatum, which for the Eastern part of the Empire dates to ca. 401, the diocese came under a vicarius of the praetorian prefecture of the East, with the title of praefectus augustalis, and included six provinces:
Aegyptus, originally established in the early 4th century as Aegyptus Iovia, under a praeses
Augustamnica, originally established in the early 4th century as Aegyptus Herculia, under a corrector
Arcadia, established ca. 397 and having previously briefly listed in the 320s as Aegyptus Mercuria, under a praeses
Parallel to the civil administration, the Roman army in Egypt had been placed under a single general and military governor styled dux in the Tetrarchy. Shortly after the creation of Egypt as a separate diocese, the post evolved into the comes limitis Aegypti, who was directly responsible for Lower Egypt, while the subordinate dux Thebaidis was in charge of Upper Egypt. In the middle of the 5th century, however, the latter was also promoted to the rank of comes. The two officers were responsible for the limitanei troops stationed in the province, while until the time of Anastasius I the comitatensesfield army came under the command of the magister militum per Orientem, and the palatini under the two magistri militum praesentales in Constantinople. The comes limitis Aegypti enjoyed great power and influence in the diocese, rivalling that of the praefectus augustalis himself. From the 5th century, the comes is attested as exercising some civilian duties as well, and from 470 on, the offices of comes and praefectus augustalis were sometimes combined in a single person. This tendency to unite civil and military authority was formalized by Justinian I in his 539 reform of Egyptian administration. The diocese was effectively abolished, and regional ducates established, where the presiding dux et augustalis was placed above the combined civil and military authority:
dux et augustalis Aegypti, controlling Aegyptus I and Aegyptus II
dux et augustalis Thebaidis, controlling Thebais superior and Thebais inferior
Augustamnica I and Augustamnica II were likewise probably — the relevant portion of the edict is defective — were placed under a single dux et augustalis
in the two Libyan provinces, the civil governors were subordinated to the respective dux
Arcadia remained under its praeses, probably subordinated to the dux et augustalis Thebaidos, and a dux et augustalis Arcadiae does not appear until after the Persian occupation of 619–629.
