The bishopric of Kos was a suffragan of the metropolitan see of Rhodes. Its bishop Meliphron attended the First Council of Nicaea in 325. Eddesius was one of the minority Eastern bishops who withdrew from the Council of Sardica in about 344 and set up a rival council at Philippopolis. Iulianus went to the synod held in Constantinople in 448 in preparation for the Council of Chalcedon of 451, in which he participated as a legate of Pope Leo I, and he was a signatory of the joint letter that the bishops of the Roman province of Insulae sent in 458 to Byzantine Emperor Leo I the Thracian with regard to the killing of Proterius of Alexandria. Dorotheus took part in a synod in 518. Georgius was a participant of the Third Council of Constantinople in 680–681. Constantinus went to the Photian Council of Constantinople (879). Under Byzantine rule, apart from the participation of its bishops in councils, the island's history remains obscure. It was governed by a "droungarios" in the 8th–9th centuries, and seems to have acquired some importance in the 11th and 12th centuries: Nikephoros Melissenos began his uprising here, and in the middle of the 12th century, it was governed by a scion of the ruling Komnenos dynasty, Nikephoros Komnenos.