The benthic Polychaeta (Annelida) collected in the course of the 1985 WIWO/RIN/NIOZ research project on the biology of waders on the Banc d’Arguin, 150 km south of Nouadhibou (Mauritania) are studied. The collection consists of 321 specimens belonging to 27 species, of which nine have not been reported previously from Mauritania, together with Marphysa mauritanica nov. spec.