An eplzootic of high mortality of the hard clam Meretrix meretrix occured along the seabeach of the East China Sea, art investigating had been car-ried out during the period of August to October 1988. The diseased clam looked verythin, it could not sink into muddy sand and its shells loosely closed. There was noresidual of food in the digestive tract. The body colour became to light red. Para-sitic copepoda were found in the mantle cavity. Pathogenic bacterium was isolated from the diseased clam. Infection experi-ments ascertained that the bacteria were the cause of the disease.The isolates showed Gram-negative short, rods with a single polar flagellum. All strains grew well andformed large yellow colonies on TCBS ager and swarmed strikingly on marine agar.They produced acid but no gas from glucose. Tests for the decarboxylation of lysineand ornithine were positive and arginine negative. They could net grow in peptonewater lacking NaCl but grew well in peptone water containing 10% NaCl. They couldnormally grow at 43℃. The organisms were sensitive to vibriostatic agent 0/129(150 μg) and novobiocin (5μg). Thus the pathogenic bacterium was identified as Vibrio alginolyticus. The bacteria grew in 1% peptone water at optimum temperaturesof 20-37C, at optimum NaCl of 1-8% and at optimum pH from 6 to 10 respectively. Electron microscope examinations of the histological ultrathin sections of the diseased clam demonstrated the presence of rod-shaped pathogenic bacteria in af-fected intestinal epithelial cells and liver cells.The pathogen multiplied and formedcolonies in the cytoplasm of intestinal epithelial cells. Nucleus degenerated and waspushed to the margin of the cells. Microvilli and cytochondriomes in the epithelialcells were destroyed. The digestive tube and liver tissues of the host become necro-sis and histolysis.