Some Cistercian monks coming from the Escale-Dieu settled in 1151 on the banks of river Baïse, on lands given by Forton del Tilh. This one gave two or three places in order to raise a barn. The abbot had to give in return 10 sous morlas but also one horse.
Monks soon cleared the estate, diverted the river to create an island on which they put up the abbey. Some donations permitted the construction of the church of Our-Lady between 1151 and 1175. From the beginning, Flaran was very important and its territory was huge.