The Danube delta hosts a very diverse population of Phragmites australis that has attracted the interest of generations of scientists. In this paper we focus on the genetic diversity of this population and how such variation is reflected in the morphology, cytology and physiological response of its genotypes. A crucial genetic trait that makes the P. australis population in the Danube delta a focal for the study of the evolutionary history of the genus Phragmites is its variation in chromosome numbers and ploidy levels co-existing next to each other. We review the literature from 1965 to today and explore the genetic relationships among the Romanian genotypes within the delta population, and within the Phragmites genus, on the base of molecular data obtained from chloroplast DNA sequencing and nuclear DNA markers (microsatellites and AFLPs). Even though cell size often increases with ploidy level, cytological variation appears not to
be the only factor explaining differences in physiology and size between Romanian ?fine? and ?giant? common reeds. The presence of tall maternal lineages and their possibility to hybridize provides an alternative explanation to the phenotypic variation pattern observed in this region. A specifically designed study of genetic variation in the delta will reveal the genetic dynamics within this special population and the processes driving the evolution in the genus.