Formation and development of hyperhydric tissue (HHT) were investigated morphologically and anatomically in Populus tremuloides seedlings flooded for 5, 9, 15,22,30, 45, and 80 days. HHT was initiated after 5 days of flooding (DF) by swelling of the filling tissue of lenticels, probably by water intake. At the same time, cell division was initiated in the phellogen and phelloderm of lenticels. Repeated divisions'of the phellogen of flooded lenticels produced long files of cells that pushed the filling tissue outwards. After 9 DF the activity of the phellogen extended beyond the lenticels. When large, extensive areas of phellogen were involved, HHT formed patches of short tissue covering most of the stem surface. When the activity of the phellogen was restricted to a small area, long columns of HHT were produced instead. In one case, in a stem flooded for 80 days, the formation of a new phellogen immediately below the old one was observed. Cells produced centrifugally by the active phellogen of flooded seedlings were thinwalled, not suberised, without nuclei, radially elongated or with irregular shape, firmly connected by their tangential walls, but with only a few points of contact with neighbouring cells by their radial walls, mainly by knoblike projections. After 22 DF the cortical parenchyma and rays of the secondary phloem started to take part in HHT formation, producing new, larger cells, rich in starch grains, with large aerenchyma spaces that greatly increased bark thickness and porosity.