The marine bacterium strain KMM 1376T was isolated from gill tissue of the ascidian Halocynthia aurantium, an inhabitant of the coastal waters of the Sea of Japan. Strain KMM 1376T is an aerobic, gram-negative, non-motile, encapsulated, non-pigmented, slightly halophilic bacterium that is characterized by fimbria-like structures, growth in 0.5-15% NaCl at 7-35 degrees C and absence of acid production from many carbohydrates. The DNA G+C content is 54 mol%. The main fatty acids are C16:0, C16.1omega7c and C18:1omega9c. Comparative 16S rDNA sequence analysis of strain KMM 1376T revealed 92.7-95.3% sequence similarity to members of Halomonas. Based on phenotypic and phylogenetic characteristics, it is proposed that the novel isolate be classified as Halomonas halocynthiae sp. nov., with the type strain KMM 1376T (= DSM 14573T).