The floristic knowledge of benthic marine algae of southern Italy appears irregular and not yet complete: up to now, in fact, there are large areas not sufficiently studied (e.g. coast of Calabria and Apulia). Nevertheless the availability of quite complete floras of some areas: gulf of Naples, W-Sicily, the Straits of Messina, the northern part of the eastern Sicilian coast, the southern part of the eastern Sicilian coast and the island of Linosa, permits to get an idea of general characteristics of algal flora of this region. This flora appears very rich of species (over 650). The degree of floristic similarity among ten Mediterranean areas (the above mentioned six areas of southern Italy as well as Catalan coast, cote des Alberes, Adriatic sea and coast of Greece) was established. A hierarchical clustering of these floras was carried out. The results are expressed in a dendrogram. A noticeable similarity among the floras can be drawn from the dendrogram since they are linked above the 50% leve...