Structure of the hardwood floodplain forest in the National Nature Reserve Lyon near the Danube and in the cadaster of Čičov has been studied by means of classical dendrometrical and dendrological inventory of both tree and shrub layers. The origin of old growth is natural and it is probable, that the greatest part of trees comes from spontaneous regeneration after the disasterous flood in 1899. In the tree layer, domestic tree species, typical of the hardwood floodplain forest ecosystems prevail; none of the species is dominating while Quercus robur L., Fraxinus angustifolia V a h 1, Populus nigra L., P. alba L., Ulmus laevis P a 11, and Salix alba L. are mixed almost regularly in the area of the research plot. Aproximately 80 percent of trees are with damaged crowns (partially dry, dry tops etc.), the best health status parameters are with individuals of Fraxinus angustifolia V a h 1 and Ulmus laevis P a 11. Vigorous shrub layer is of high biodiversity; typical shrub species do not hinder the natural regeneration of Fraxinus angustifolia V a h l which forms a very perspective part of the ecosystem. Special study on ash regeneration under old trees has been performed and evaluated in this study. Beginning of the tree layer destruction is obvious and this causes probably excellent conditions for the ash regeneration.