AbstractUntil recently,Festuca arietinawas practically an unknown species in the flora of Eastern Europe. Such a situation can be treated as a consequence of insufficient studying ofFestuca valesiacagroup species in Eastern Europe and misinterpretation of the volume of some taxa. As a result of a complex study ofF. arietinapopulations from the territory of Ukraine (including the material fromlocus classicus), Belarus and Lithuania, original anatomy, morphology and molecular data were obtained. These data confirmed the taxonomical status ofF. arietinaas a separate species. Eleven morphological and 12 anatomical characters, ITS1-5.8S-ITS2 cluster of nuclear ribosomal genes, as well as the models of secondary structure of ITS1 and ITS2 transcripts were studied in this approach. It was found for the first time thatF. arietinais hexaploid (6x = 42), which is distinguished from all the other narrow-leaved fescues by specific leaf anatomy as well as in ITS1-5.8S-ITS2 sequences. Molecular data indicating possible hybridogenous origin ofF. arietina, fall in line with the anatomical-morphological data and explain the tendency toward sclerenchyma strands fusion with formation of a continuous ring inF. arietina, as well asF. arietinaecological confinement to psammophyte biotopes.