This study was performed to ascertain the relationships among clionid and spirastrellid species from the Mediterranean Sea, based on phylogenetic criteria. A matrix of 34 taxa and 20 characters was analysed. Cladistic analysis using PAUP produced 12 equally parsimonious trees of 54 steps (CI=0.648, RI=0.865). Resolution was high in all parts of the strict consensus tree except in those which involved relationships among the most characteristic
clionid species: those having only tylostyles as megascleres and, if microscleres are present, these being spirasters. Six monophyletic groups, three of them represented by monospecific clades, appear in the totally resolved part of the trees, which supports their separation into different genera. On the basis of the results obtained, the species at present within the genus Cliona have been allocated to five different genera: Scantilletta , Pione, Volzia, Bernatia
and Cliona, while Dotona and Cliothosa are maintained as separate genera. Scantilletta and Pione are genera erected by de Laubenfels (1936 Pap. Tortugas Lab. 30, 1–225) and Gray (1867 Proc. Zool. Soc. Lond. XXXII, 492–558) respectively, and whose diagnoses, are amended here. Volzia and Bernatia are proposed as new genera to include Cliona albicans
and Cliona rovignensis, and Cliona vermifera, respectively. Thus, the genus Cliona clearly appears in our analysis as a polyphyletic group. It is only maintained provisionally to harbour species whose position cannot be clearly ascertained from the information at hand. Skeletal characters such as spicule types and skeletal arrangement have proven to be useful in discriminating some genera and families but they do not completely resolve the phylogeny of this group of species. Biochemical, genetic, cytological or reproductive information, when available, may help in the resolution of the phylogenetic tree.