Field investigations in two lymnaeid habitats located along river banks were carried out over three years to study snail colonization in relation to water-level variations, to determine the number of annual generations, and to analyse the ability of G. truncatula to emerge from water. As these river banks were located 6.7 km upstream from a dam, water recedes each day, thus permitting the study of snails on banks for several hours per day. From March to December, snails followed seasonal variations of water-level. However, only a few G. truncatula were observed following the daily retreat of water. Two annual generations of G. truncatula, the first from the onset of July and the second in December, were noted in these habitats. When water receded each day, the mean distance covered by non-aestivating snails ranged from 25 to 44 cm. However, they escaped water rise by seeking refuge in still emerged zones of their habitats. From July to October, snails following each day water receding showed a significantly quicker growth than that of G. truncatula which only moved in the same zone of their habitat. Populations of G. truncatula living along these river banks would be completely adapted to their habitat. The escape of snails from water rise between July and October might correspond to a physiological reaction of survival when a too rapid increase of water-level occurs.