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, côte des Albères, 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% level of importance, excepting that of Greece alone (about 45%). A clustering, based on the chorological spectrum of each flora, was also carried out and then depicted in a dendogram which shows a much more marked degree of similarity among the floras. However, two groupes can be individuated, the first on encompassing the flora of the Adriatic sea as well as those of southern Italy (except Messina), the second one all the others.