Four bacterial strains isolated from carpet-shell clams (Ruditapes decussatus) and manila clams (R. philippinarum) exhibiting Brown Ring Disease (BRD) symptoms in Galicia (Northwest of Spain) were characterized using biochemical and serological techniques. Moreover their pathogenicity for clams and resistance against clam hemolymph were determined. All of the Vibrio isolates exhibited biochemical characteristics typical of the genus. Strains GR1703RP and GR6301RP, isolated from R. philippinarum showing signs of BRD, shared phenotypical and antigenic characteristics and bacterial extracellular products enzymatic activities with the Vibrio tapetis. Strains GR0705RD and GR0202RD, isolated from R. decussatus showing BRD signs, although biochemically similar to V. tapetis, did not show positive agglutination with V. tapetis antiserum. The virulence of the bacteria varied depending on the bacterial strain and clam species assayed. Mortalities were recorded for a 30-day period, reaching values ranging from 6 to 80%. Strain GR0202RD was the most virulent for R. philippinarum with a cumulative mortality rate of 45%, while the strain GR0705RD induced the highest prevalence of BRD signs, affecting 75% of the survivors. In the case of R. decussatus, the four bacterial strains produced a cumulative mortality rate higher than 45%. In general, bacterial strains, showed resistance to clams hemolymph. Copyright 1998 Academic Press. Copyright 1998 Academic Press