Juvenile Oyster Disease (JOD) has resulted in mass mortalities of cultured Crassostrea virginica in the northeastern United States since the 1980's. Bacteriological analyses of affected oysters have revealed a consistent association of JOD-signs and mortalities with heavy colonization by the novel species Roseovarius crassostreae. Conclusive assignment of etiology has been difficult, however, for two principle reasons. First, JOD-like mortality was reproduced in laboratory-held oysters by challenge with R. crassostreae, but the typical disease signs were not present. Further, because R. crassostreae had only been detected in oyster populations after the onset of JOD, the possibility remained that it was a secondary colonizer. In this study, we document the ability of R. crassostreae to induce JOD-like conchiolin deposition in oysters maintained in laboratory aquaria. Further we report the isolation of R. crassostreae from a first-year crop of C. virginica concurrent with the development of microscopic mantle lesions, which are the first pathological indication of JOD. Consistent with previous studies, these lesions preceded overt JOD-signs and mortalities by less than two weeks. Based on the accumulated evidence, it is now possible to rule out a secondary role for R. crassostreae as a colonizer of already diseased animals, and we conclude that R. crassostreae is the etiological agent of JOD. To avoid potential confusion with other diseases of juvenile oysters, we also propose that JOD be renamed “ Roseovarius Oyster Disease” (ROD).