A Gram-reaction-negative, aerobic, rod-shaped (1.2-1.6 ?m��0.6-0.8 ?m), flagellated and motile marine bacterium, designated MEBiC06243(T), was isolated from a sediment collected at Daebu Island in the Yellow Sea (37�X 20' N 126�X 41' E), Korea. The 16S rRNA gene sequence analysis revealed that strain MEBiC06243(T) showed high similarity with Neptunomonas naphthovorans NAG-2N-126(T) (96.3%). Growth was observed at 10-39 �XC (optimum 29 �XC), at pH 6.0-9.0 (optimum pH 7) and with 0-7% (optimum 2.5%) NaCl. The predominant cellular fatty acids were C(10:0) 3-OH (6.1%), C(12:0) (5.8%), C(16:0) (30.5%), C(18:1)�s7c (21.6%) and summed feature 3 (comprising C(15:0) 2-OH and/or C(16:1)�s7c; 30.7%). The DNA G+C content was 41.4 mol%. The major respiratory quinone was Q-8. Phosphatidylethanolamine, phosphatidylglycerol, diphosphatidylglycerol, two unidentified lipids, one unidentified aminophospholipid and three unidentified aminolipids were detected as major polar lipids. On the basis of this polyphasic taxonomic data, strain MEBiC06243(T) should be classified as a novel species of the genus Neptunomonas proposed as Neptunomonas acidivorans sp. nov. The type strain is MEBiC06243(T) (=KCCM 42975(T) =JCM 18291(T)). An emended description of the genus Neptunomonas is also given.