Some important morphological features, which are discussed here, point out that the Stenasellids (Crustacea Isopoda Asellota) must be considered as a true family (Stenasellidae), independent from the Asellidae. A definition and a renewed diagnosis of the Stenasellidae Dudich, 1924, are given. Their relationships must be pursued, especially in the marine Parastenetroidea and in the psammic Microcerberidae. Until 1938, the group was known only from subterranean waters of southern Europe. Now, several genera and many thermophile species from north-tropical underground waters have been discovered in Africa (5 gen., 12 sp.), Asia (1 gen., 2 sp.) and Central America (1 gen., 4 sp.). The Stenasellids are very active burrowers. Such a behaviour explains how their phyletic lines had colonized the continental underground waters, by migrations from the littoral gravels to the underflow of rivers, phreatic alluvial waters and fìnally, to the karstic waters. The typical medium for the life of the group is represented by the phreatic zones of African shields arenas. In European phyletic lines, the speciation seems to be linked with tertiary subsidences (within the Tyrrhenian area, for the line of Stenasellus virei). The European species which have survived quaternary glaciations may have diversified themselves (rising of subspecies), recolonizing newly vacant biotopes in postglacial ages.