Ancient Diocese of Saint-Malo Wikipedia
The former Breton and French Catholic diocese of Saint-Malo existed from at least the seventh century until the French Revolution. Its see was at Aleth, to a point in the twelfth century, when it moved to Saint-Malo. Its territory extended over some of the modern departments of Ille-et-Vilaine, Côtes-d'Armor and Morbihan. Until the 860s it was often termed bishopric of Poutrocoet.