During spring 1975, dragonflies were collected in various localities in the Western (Atlantic) Sahara. Great faunal heterogeneity was found while moving from North to South. The Moroccan Sahara has a fauna which does not differ from that of North Atlantic Morocco; the fauna of Rio de Oro is very poor, but has even now been insufficiently "sampled”; the fauna of Mauretania is purely Ethiopian, but almost all species known from that country (ten out of twelve) also occur in Morocco and Algeria. Analysis of these dragonfly faunas and a comparison with other aquatic groups, confirm the theory that the boundary between the F.thiopian and Palaearctic domains runs through the desert, somewhere around the latitude of the Tropic of Cancer. It is also concluded that the expansion of the Ethiopian fauna, following the Eocene emergence of the Sahara, must have been more important than that of the Palaearctic fauna, since many Ethiopian elements still occur along the shores of the Mediterranean, but no Palaearctic species is yet known which has succeeded in establishing itself south of the Tropic of Cancer.