A Gram-negative, non-motile and rod-, oval- or coccoid-shaped strain, DSW-17(T), was isolated from seawater of the East Sea, Korea, and subjected to a polyphasic taxonomic study. Strain DSW-17(T) grew optimally at pH 7.0-8.0 and 25 degrees C. It contained Q-10 as the predominant ubiquinone and C(18 : 1)omega7c as the major fatty acid. Major polar lipids were phosphatidylcholine, phosphatidylglycerol, phosphatidylethanolamine and an unidentified glycolipid. The DNA G+C content was 65.2 mol%. Phylogenetic analysis based on 16S rRNA gene sequences showed that strain DSW-17(T) was phylogenetically most closely affiliated to the genus Jannaschia. Strain DSW-17(T) exhibited 16S rRNA gene sequence similarity values of 96.5 % with the type strains of three recognized species of the genus Jannaschia. DNA-DNA relatedness data and differential phenotypic properties, together with the phylogenetic distinctiveness, demonstrated that strain DSW-17(T) is distinguishable from the recognized species of the genus Jannaschia. On the basis of phenotypic, phylogenetic and genetic data, strain DSW-17(T) was classified in the genus Jannaschia as a member of a novel species, for which the name Jannaschia donghaensis sp. nov. is proposed. The type strain is DSW-17(T) (=KCTC 12862(T)=JCM 14563(T)).