Since 1988, juvenile oyster disease (JOD) has resulted in high seasonal losses of cultured Eastern oysters (Crassostrea virginica) in the Northeast. Although the cause of JOD remains unknown, most evidence is consistent with either a bacterial or a protistan etiology. For the purpose of discerning between these hypotheses, the antibacterial antibiotics norfloxacin and sulfadimethoxine-ormetoprim (Romet-B) were tested for the ability to delay the onset of JOD mortality and/or reduce the JOD mortality of cultured juvenile C. virginica. Hatchery-produced C. virginica seed were exposed in triplicate groups of 3,000 animals each to either norfloxacin, sulfadimethoxine-ormetoprim, or filter-sterilized seawater (FSSW) and deployed in floating trays on the Damariscotta River of Maine on 17 July 1997. Each week thereafter, a subset of animals from each group was reexposed to the assigned treatment. Repeated immersion in either a sulfadimethoxine-ormetoprim or a norfloxacin solution resulted in a delay in the onset of JOD mortality in treated animals and reduced weekly mortality rates. Weekly treatments with either norfloxacin or sulfadimethoxine-ormetoprim also resulted in a statistically significant reduction in cumulative mortality (55 and 67% respectively) compared to animals treated weekly with FSSW (81%) or those that had received only a single treatment with either norfloxacin, sulfadimethoxine-ormetoprim, or FSSW (77, 84, and 82%, respectively). Bacteriological analyses revealed a numerically dominant bacterium in those animals with obvious signs of JOD. Sequence analysis of the 16S rRNA gene from these bacteria indicates that they are a previously undescribed species of marine -proteobacteria.