Can we figure out what makes a culture or a literature being Celtic or not ? Besides linguistic considerations – Breton is obviously a Celtic language – it appears quite tricky to tell the Celtic features from the others in a given culture. That is to say not just a few frozen scattered figures (such a Breton and Celtic feature as King Arthur left very few traces in medieval Breton literature and further) but dynamic features that could be considered as part of the Breton cultural system. This presentation, dealing with the (paradigmatic?) case of the city of Ys, aims to point out what could be considered a Celtic feature. The topic is well-known : the rise and fall of an impressive city built below sea level off the coast of Brittany. The old king Gralon and the leading part of his daughter, Dahud ,who will become finally a mermaid. We’ll set out how this topic, that has been shared by a few Celtic cultures, made its way through the Breton repertoire from a medieval mystery play to modern folklore and finally contemporary literature and so became a dynamic (canonical?) stratum in Breton literature.