Growth form, life cycle and the modes of vegetative reproduction and overwintering of 18 Potamogeton species were revealed by an extensive field survey conducted throughout Japan. In the study of growth form, the development of floating leaves and the formation of land forms were dealt with. Results showed remarkable plasticity in growth form of P. distinctus and P. malaianus, which was thought to be a significant adaptation of these species to occupy unstable habitats liable to water drawdown. The formation, dormancy and germination of the turions, especially of axillary or apical ones, were compared and discussed in relation to the life cycle of the species. Different degrees of plasticity in growth form and the development of diverse turion habits were considered to be significant features in the ecological diversification of Potamogeton.