An orange, rod-shaped, gliding bacterium, designated strain PSC33(T), was isolated from the gut microflora of a mussel collected from Gwangyang Bay, South Sea (Republic of Korea). Cells were Gram-reaction-negative, strictly aerobic, and catalase- and oxidase-positive. The major fatty acids were iso-C(15:0), iso-C(17:0) 3-OH, iso-C(15:1) G, C(15:0) 3-OH and iso-C(17:1)ω9c. The only isoprenoid quinone was menaquinone-6 (MK-6). The DNA G+C content of strain PSC33(T) was 37.9 mol%. A phylogenetic tree based on 16S rRNA gene sequences showed that strain PSC33(T) formed an evolutionary lineage within the radiation encompassing members of the genus Aquimarina with Aquimarina macrocephali JAMB N27(T) as its nearest neighbour (96.3% sequence similarity). A number of phenotypic characteristics distinguished strain PSC33(T) from recognized members of the genus Aquimarina. On the basis of the data presented in this study, strain PSC33(T) is considered to represent a novel species of the genus Aquimarina, for which the name Aquimarina mytili sp. nov. is proposed. The type strain is PSC33(T) ( = KCTC 23302(T) = JCM 17454(T)). An emended description of A. macrocephali is also provided.