Most species of terrestrial Isopoda require a high degree of humidity to survive, but only a small number of species are known to live strictly or temporarily in aquatic environment. Many of the aquatic Oniscidea belong to the families Trichoniscidae, Styloniscidae (Synocheta), Olibrinidae and Philosciidae (Crinocheta), most of which inhabit subterranean waters. The aquatic species often show peculiar morphological characters and have been considered as primitive taxa compared with related terrestrial ones. Our study deals with species belonging to two genera (Alpioniscus and Utopioniscus - fam.Trichoniscidae) from Sardinia Island. At present Alpioniscus includes two terrestrial species in Sardinia, the subterranean A. fragilis and the endogean A. thanit, while Utopioniscus is known with one aquatic species (U. kuehni) from two anchialine caves. For its morphology and aquatic life U. kuehni was considered to be the most primitive representative of the family Trichoniscidae. Recent investigations in Sardinian caves revealed the presence of U. kuehni also in subterranean freshwater streams and of another two new aquatic species which show intermediate morphological characters between Alpioniscus spp. and U. kuehni. A molecular analysis,carried out using new designed primers for the mtDNA COI gene, shows that the aquatic species of Trichoniscidae are included in the same clade with Alpioniscus spp., suggesting that U. kuehni is not the most primitive Trichoniscidae but rather a species of Alpioniscus secondarily returned and morphologically adapted to an aquatic way of life. Similar examples are present in other oniscidean genera such as Trogloniscus (Styloniscidae) from China and Haloniscus (Philosciidae) from Australia.