Lethal bacterial infections of a variety of hatchery-spawned bivalve mollusk larvae and juveniles have been studied. The symptoms of the disease and the course of the infection are described. Four biotypes and five antigenic types of bacteria, pathogenic for the larvae of five species of bivalve mollusks, were isolated and described in some detail. All are gram-negative motile rods. Comparative studies were made of a fairly large number of similar bacteria isolated from presumably normal marine fauna. None of these was pathogenic for the bivalve larvae nor did they have antigens in commoni with the pathogenic group. The four biotypes had a number of characteristics in common that rarely were present in other cultures from marine fauna. Several antibiotic preparations proved to be of value in the treatment and control of the infection.

The hemolymph of 290 freshly collected blue crabs from Chincoteague Bay, Va., was sampled over a 15-month period from August 1968 through November 1969 and most probable numbers of bacteria were determined by tube dilution. The hemolymph of 18% of all crabs sampled was found to be sterile, with 16% sterility in summer and 23% in winter samples. Despite individual variations, male crabs as a group had a higher bacterial hemolymph burden than females, and among both sexes summer counts were higher than winter. The hemolymph of crabs with missing appendages had significantly higher counts than uninjured crabs. The annual mean hemolymph most probable numbers per ml was 2,756 for males, 1,300 for females, and 1,876 for both sexes. The higher bacterial levels found in the hemolymph of male crabs may, in part, be explained by the fact that males, which predominated in the summer samples, had a higher incidence of injury and missing appendages than did females.

The etiological agents of bacillary necrosis, a disease of larval and juvenile bivalve mollusks, are designated members of the genus Vibrio.