Eunice sebastiani, a new species here described, belongs to the group of species called "fuscus-bidentate" by Hartman, and which includes E. aphroditois Pallas. Like some of the others which belong to the same genus, it grows to a great size. It lives in almost vertical galleries, in muddy beaches, generally in the intertidal zone. It resembles E. aphroditois Pallas (as in a figure by Izuka 1912) and E. roussaei Quat. in its size and external characteristics. E. sebastiani can be distinguished from the species mentioned chiefly by the different form of the mouth parts and dorsal cirri of the anterior and median feet. The mouth parts (Fig. 2) are light grey in colour; the maxilla I ("forceps") has thin and almost cylindrical branches; maxilla II is narrow and has very long teeth. Length of one specimen with 450 segments: 208 cm; width in the anterior region 2 cm; length of gills: 1.6 cm. Colour varying from flesh-colour to dark brown, iridescent.