The Authors studied the morphological, biochemical, physico-chemical and biological characteristics of Vibrio anguillarum cultured on different growth conditions, characterized by low osmolarity and high temperature (37 degrees). One culture was subcultured for several days in tryptone soya agar with 0.5% Nacl at 37 degrees C incubation until the cell morphology was stabilized. The low osmolarity, through an osmotic shock, induced remarkable morphological modifications in the strain, evidenced by optic and electron microscopic studied; in addition SDS-PAGE analysis of saline extracts from the culture at 37 degrees C showed a specific new protein band of about 66KDa. This band was correlated with remarkable differences in outer membrane protein composition (OMPs) evidenced by Ag/Ah cross-reactions with rabbit hyperimmune sera against the modified and the reference V. anguillarum strains. Finally, the modified strain proved to be non pathogenic for trout and sea bass.