The distribution of young and old Phalaris arundinacea and Urtica dioica plants across the Lužnice River floodplain (Třeboň Biosphere Reserve, South Bohemia, Czech Republic) was studied over a three-year dry period. Plants of both species that germinated in the current year were found on sites disturbed by floods and on emerged pool bottoms. Older P. arundinacea plants were only occasionally found on unshaded sites (pool bottoms, sandy drifts in river bed). U. dioica established itself more often on sites shaded by herbaceous plants or by neighboring shrubs. The long-term survival of U. dioica on wet sites seems uncertain because this species is less tolerant of flooding than P. arundinacea. Thus episodic expansion of U. dioica into unmown parts of the floodplain is expected.