About 20 spp of marine mammals are actually known for the fauna of the north- west African coast (Rio de Oro, Mauritanie, Senegal). This large number of spp seems to be related to cold water areas connected with the 'upwelling' effect. New data, recently collected, mainly during the summer cruise (1973) of the 'Thalassa', supply new information about the distribution of some spp. Souza teuszil shows a northern limit of its range at {approx} 20 {degree}N (Banc d'Arguin) and Stenella longirostris at {approx} 16{degree}15 N. The common porpoise (Phocoena phocoena) seems to be more frequent in Mauritanian waters than in Morrocan ones, and Orcinus orca is commonly observed between Cap Blanc and Cap Timiris. In July and Sept newborn pups of the monk seal (Monachus monachus) were found at the Rio de Oro colony. Estimation of the local population there is {approx} 50, this number showing a certain stability for many years.