A Gram-stain-negative, non-flagellated, non-gliding, aerobic and ovoid or rod-shaped bacterium, designated OITF-51(T), was isolated from a tidal flat in Oido, an island of South Korea, and subjected to a polyphasic taxonomic study. Strain OITF-51(T) grew optimally at 30 degrees C, at pH 7.0-8.0 and in the presence of 2.0% (w/v) NaCl. The neighbour-joining phylogenetic tree based on 16S rRNA gene sequences revealed that strain OITF-51(T) forms a cluster with the type strains of three species of the genus Flaviramulus. Strain OITF-51(T) exhibited 16S rRNA gene sequence similarity values of 97.0, 95.8 and 95.8% to the type strains of Flaviramulus ichthyoenteri, Flaviramulus basaltis and Flaviramulus aquimarinus, respectively, and of less than 96.5% to the type strains of other recognized species. Strain OITF-51(T) contained MK-6 as the predominant menaquinone and iso-C-15 : 1 G, iso-C-15 : 0, iso-C-15 : 0 3-OH and iso-C-17 : 0 3-OH as the major fatty acids. The major polar lipids detected in strain OITF-51(T) were phosphatidylethanolamine and one unidentified lipid. The DNA G+C content of strain OITF-51(T) was 33.4 mol%, and its mean DNA-DNA relatedness value with the type strain of F. ichthyoenteri was 13 %. Differential phenotypic properties, together with its phylogenetic and genetic distinctiveness, revealed that strain OITF-51(T) is separated from recognized species of the genus Flaviramulus. On the basis of the data presented, strain OITF-51(T) is considered to represent a novel species of the genus Flaviramulus, for which the name Flaviramulus aestuariivivens sp. nov. is proposed. The type strain is OITF-51(T) (= KCTC 52657(T) = NBRC 112707(T)).