Ruppia filifolia is the seagrass with the world's southernmost distribution and a species about which even the most fundamental aspects of its biology and ecology are not known. Presence, bathymetric distribution, and biomass of R. filifolia meadows in Skyring sound (sub-antarctic ecoregion of Magallanes, Chile) were evaluated through a remotely operated vehicle (ROV) survey, and direct verification and sampling by divers. The littoral of Skyring sound is surrounded by patchy R. filifolia meadows to a depth of 5.3 m in bands 100-160 m wide. R. fillfolla achieved a biomass of 80 g dry weight m-2, a rhizomes plus roots to leaves biomass ratio of 1.6 to 1.9 and a density of 2.000 shoots m-2. Possible relevance of meadows formed by this seagrass species in the functioning of the shallow coastal ecosystem of Skyring sound is discussed. (Journal) This study was financed by project PR-344 from Ministerio de Educación de Chile, and by the >Programa de Atracción e Inserción de Capital Humano Avanzado y Programa de Atracción de Capital Humano Avanzado del Extranjero, Modalidad Estadías Cortas (2011)> of the Comisión Nacional de Investigación Científica y Tecnológica - CONICYT, Chile Peer Reviewed