A bacterial strain belonging to the genus Vibrio was involved in a recurrent outbreak of disease in Manila clam Tapes philippinarum larvae at a commercial hatchery, This vibrio, named VTP for convenience, was not related to the known pathogenic vibrios and possessed unusual characteristics, Only Manila clam larvae and postlarvae were affected by the disease, Oyster and scallop larvae were not affected by vrp, Survival of vrp was found to be of short duration in seawater but even a very small number of cells (1 cell per 5 ml) could initiate the disease, VTP did not grow on thiosulfate-citratebile-sucrose agar and was resistant to chloramphenicol. An epidemiological study utilizing bioassays revealed the presence of VTP in adult bivalves and in the pond-inlet seawater. Drying the facilities and pond used to produce phytoplankton eliminated VTP,