Cladium mariscus (L.) Pohl (Cyperaceae) is a rare species in Europe considered by several authors to be a relict of the early Holocene period. It is listed in the Red Data Book of Ukraine, Annexes of the Habitat Directive and the Bern Convention. Communities with domination of this species are included in the Green Data Book of Ukraine. Substantial differences in major ecological factors for Cladium mariscus communities in the western (carbonate bogs) and the southern (marshes and floating swamps of the northern Black Sea) regions of Ukraine were shown. The author carried out comparisons of relevés characterizing different communities with Cladium mariscus within Europe. Based on the results of TWINSPAN analysis, four associations were identified, confirmed by floristic indices and ecological data: Cladietum marisci Allorge 1921, Soncho maritimi-Cladietum marisci (Br.-Bl. & O. de Bolòs 1957) Cirujano 1980, Dorycnio recti-Cladietum marisci Gradstein & Smittenberg 1977 and Junco maritimi-Cladietum marisci (Br.-Bl. & O. de Bolòs 1957) Géhu & Biondi 1988. Thus, in addition to the association Cladietum marisci, a new one was indicated for Ukraine, Junco maritimi-Cladietum marisci.