A set of 146 Antarctic marine isolates from the Ross Sea was characterized by a combination of molecular techniques in order to determine the degree of inter- and intraspecific variability. Isolates were analyzed by amplified rDNA restriction analysis (ARDRA) using the tetrameric enzyme AluI, resulting in 52 different groups, corresponding to at least 52 different bacterial species, indicating a high degree of interspecific variability. The phylogenetic position of bacteria belonging to some ARDRA groups was obtained by sequencing of 16S rDNA. Random amplified polymorphic DNA (RAPD) analysis, carried out on the largest ARDRA groups, revealed a high intraspecific genetic variability, too. The analysis of plasmid content revealed the existence of horizontal gene transfer between strains belonging to the same and to different species. A comparison of the whole body of morphological, physiological and biochemical data was finally carried out.