An alpha-proteobacterium has been identified which is believed to be the causative agent of juvenile oyster disease (JOD). Since its first isolation in 1997, the bacterium has been recovered as the numerically dominant species from JOD-affected animals throughout the north-eastern United States (Maine, New York and Massachusetts). Colonies are usually beige to pinkish-beige, although the majority of isolates recovered in 2003 from an epizootic in Martha's Vineyard, Massachusetts, produce colonies with a greenish-yellow appearance. The cells are Gram-negative, aerobic, strictly marine and rod or ovoid in appearance. They are actively motile by one or two flagella, but cells are also observed to produce tufts of polar fimbriae. The principal fatty acid in whole cells is C(18:1)omega7c and other characteristic fatty acids are C(16:0), C(10:0) 3-OH, 11-methyl C(18:1)omega7c and C(18:0). Almost without exception, isolates have 16S rRNA gene sequences that are 100% identical to each other. Phylogenetic analyses place the organism within the Roseobacter clade of the alpha-Proteobacteria, with moderate bootstrap support for inclusion in the genus Roseovarius. DNA-DNA relatedness values from pairwise comparisons of this organism with the type species of the genus (Roseovarius tolerans) and the only other described species in this genus, Roseovarius nubinhibens, were 11 and 47%, respectively. Phenotypic and biochemical dissimilarities also support the assignment of this bacterium to a novel species. The name Roseovarius crassostreae sp. nov. is proposed, with the type strain CV919-312(T) (=ATCC BAA-1102(T)=DSM 16950(T)).