Two Gram-negative, motile, non-spore-forming, rod-shaped organisms, strains SW-117(T) and SW-120(T), were isolated from sea water of the Yellow Sea in Korea and subjected to a polyphasic taxonomic study. Strains SW-117(T) and SW-120(T) simultaneously contained both menaquinones (MK) and ubiquinones (Q) as isoprenoid quinones; the predominant menaquinone was MK-7 and the predominant ubiquinones were Q-7 and Q-8. The major fatty acid detected in the two strains was iso-C(15 : 0). The DNA G+C content of strains SW-117(T) and SW-120(T) was 51 and 54 mol%, respectively. Phylogenetic analyses based on 16S rRNA gene sequences showed that strains SW-117(T) and SW-120(T) fall within the radiation of the cluster comprising Shewanella species. Strains SW-117(T) and SW-120(T) showed a 16S rRNA gene sequence similarity of 97.4 % and a DNA-DNA relatedness level of 10.1 %. Strains SW-117(T) and SW-120(T) exhibited 16S rRNA gene sequence similarity levels of 93.8-98.5 % and 92.4-97.0 %, respectively, to Shewanella species. Strain SW-117(T) exhibited DNA-DNA relatedness levels of 8.3-20.3 % to the type strains of six phylogenetically related Shewanella species. On the basis of phenotypic, phylogenetic and genetic data, strains SW-117(T) and SW-120(T) were classified in the genus Shewanella as two distinct novel species, for which the names Shewanella marisflavi sp. nov. (type strain, SW-117(T)=KCCM 41822(T)=JCM 12192(T)) and Shewanella aquimarina sp. nov. (type strain, SW-120(T)=KCCM 41821(T)=JCM 12193(T)) are proposed, respectively.