Ever since the early colony, the settlement of the islands of the St. Lawrence has withstood sharp changes. The evolution of the population of the six settled islands (or group of islands) lying between Montréal and Anticosti underlines the nature of the dependency under which these localities have survived as actors and spectators of the development of the laurentian axis. The case of Anticosti island is probably the most eloquent. Although less spectacular, the growth of the population of the other islands {Cent-lles of lake St. Peter, lie d'Orléans, Ile aux Grues, lie aux Coudres and lie Verte) has also been erratic. All of them, at one point or another (examples : lie d'Orléans around 1871 ; Ile aux Coudres around 1956) have seen their rural population reach a level of saturation generally followed by a sharp drop in the total population of the island community. Only the lie d'Orléans and Cent-Îles have been the object of a new demographic « take-off » thanks to a readaptation of their function. The comparative study of the recent demographic history of the two municipalities of the Cent-Îles group contributes to illustrate the paradox of insularity : the permanency of the insular settlements of the St. Lawrence can only be safeguarded under conditions implying the erosion of the cultural characters that define the originality of these insular localities.