Several virulence factors involved in the potential pathogenic capacity of Vibrio P1, the causative agent of brown ring disease (BRD) affecting cultured Manila clam adults (Ruditapes philippinarum), have been evaluated in comparison with other strains of several Vibrio species isolated from diseased clams. The importance of bacterial cell surface associated properties as virulence factors has been studied considering both non-specific and specific bacterial adhesion to clams. Vibrio P1 showed moderate hydrophobicity, but high affinity to bind to Congo Red dye and the presence of appendages, characterized as fimbriaeor pili. All the strains of Vibrio P1 secreted haemolysis and cytotoxins, and were also strong exotoxin producers. The presence of a large 49.2-MDa plasmid in all the strains of Vibrio P1 may be used as an epidemiological marker, but its involvement in pathogenic mechanisms has not yet been established. Although in some Vibrio strains, iron-acquisition systems play an essential role in their pathogenicity, they do not seem to be an important factor in Vibrio P1, since this pathogen lacks siderophore-mediated iron transport mechanisms.