Abstract. Devillers-Terschuren, J. & Devillers, P.- The arachnitiform Ophrys of the Western Mediterranean basin [Natural. belges (Orchid. 2) 69 (1988): 98-112]. The arachnitiform Ophrys of the Mediterranean basin can be arranged in three groups. The first is formed of early-flowering plants with characters entirely derived from those of the Ophrys sphegodes complex; it includes O. arachnitiformis, O. praecox, O. exaltata and probably O. cephalonica. The second, composed of species which ally the characters of the O. sphegodes group with those of other complexes, comprises O. tyrrhena, O. morisii and probably O. archipelagi. The third is constituted by three late-flowering species, O. splendida, O. aveyronensis and a taxon of the Castilian plateaux, described here as Ophrys castellana, linked to humid habitats unusual for the genus. It is suggested that the origin of the first group is to be sought in the geographical fragmentation of the range of an ancestral arachnitiform species and that of the second in the interaction of species of the first group with other complexes, while the species of the third are independently derived from the first group or from forms of the O. sphegodes group. Attention is drawn to the conservation problems that may arise for species of the group.