Water-lilies (the genus Nymphaea) are one of the most ancient groups of aquatic plants. Two species (N. alba and N. candida) are native to the flora of the Czech Republic, both are critically endangered. Morphological similarities, large phenotypic variation and putative interspecific hybridization pose difficulties for species identification. The amount of nuclear DNA was found to be a reliable characteristic that allows not only species but also their hybrids to be re - cognized. Native species are mainly threa - tened by the loss of suitable habitats and planting of garden cultivars in natural sites. and Klára Kabátová, Petr Vít, Jan Suda.