During several cruises carried out by the Ifremer (Institut fran<;ais de recherche pour l'exploitation de la mer) with the submersile "Nautile" at different hydrothermal sites and cold seeps, an important collection of anthozoans - mainly actiniarians - was sampled. Additional material was collected by an American cruise with the submersile "Alvin". The goal of the present contribution is to describe part of this actiniarian fauna inhabiting three chemoautotrophic sites, one of them in the Mid-Atlantic Rise and the other two in the Eastern Pacific. A new genus and species of actinostolids is proposed after the study of specimens collected at hydrothermal vents in the Guaymas Basin (East Pacific Rise, Gulf of California, 27°00'N-111°24'W, 2020 m). Furthermore, some specimens collected in cold seeps along the MAT (Middle America Trench, off Mexico, 18°21'-20°01'N 104°21'-106°17'W, 3354-3795 m) are reported as Hadalanthus aff. knudseni. This material is discussed and compared with the existing type material of H. knudseni Carlgren, 1956, and Tealidium jungerseni Carlgren, 1921. In addition, some specimens of Maractis rimicarivora Fautin & Barber, 1999, were collected at hydrothermal vents of the Mid Atlantic Ridge Snake Pit (23°22.9'N-44°55.8'W, 3490 m) site. This species is collected for the first time after its original description at Trans-Atlantic Geotraverse (TAG, 26“08'N 44°49.6'W, 3650 m). Some ecological remarks are given from in situ and video observations

29 pages, 16 figures, 5 tables

Peer reviewed