A Gram-negative, non-motile, rod-shaped bacterial strain, SW-255(T), was isolated from seawater from Hwajinpo, on the coast of the East Sea, Korea, and subjected to a polyphasic taxonomic study. Strain SW-255(T) grew optimally at pH 7.0-8.0 and 37 degrees C in the presence of 2 % (w/v) NaCl. It contained Q-10 as the predominant ubiquinone and C(18 : 1)omega7c as the major fatty acid. The DNA G+C content was 67.0 mol%. A neighbour-joining phylogenetic tree based on 16S rRNA gene sequences showed that strain SW-255(T) is phylogenetically closely related to the genera Ruegeria and Silicibacter of the Alphaproteobacteria. The levels of 16S rRNA gene sequence similarity between strain SW-255(T) and the type strains of Ruegeria atlantica and two Silicibacter species were in the range 95.8-96.2 %. A phylogenetic tree based on gyrB sequences showed that strain SW-255(T) forms a distinct evolutionary lineage within the Alphaproteobacteria. Differential phenotypic properties, polar lipid profiles and DNA G+C contents, together with the phylogenetic distinctiveness, suggest that strain SW-255(T) should be distinguished from the members of the genera Ruegeria and Silicibacter. On the basis of the phenotypic, chemotaxonomic and phylogenetic data, strain SW-255(T) represents a novel genus and species, for which the name Pseudoruegeria aquimaris gen. nov., sp. nov. is proposed. The type strain of Pseudoruegeria aquimaris is SW-255(T) (=KCTC 12737(T)=JCM 13603(T)).