A novel Gram-negative, strictly aerobic, agar-hydrolysing bacterium, designated YM01(T), was isolated from seawater samples collected from the Yellow Sea (coastal region of Qingdao, PR China). Cells were rod-shaped, peritrichously flagellated and formed long chains end-to-end. The isolate had an absolute requirement for Na(+) ions, but not seawater, for growth and grew optimally at about 28 °C, in 2% NaCl and at pH 8.0-9.0. The isolate could not be cultured in marine broth 2216, but grew well on marine agar 2216. YM01(T) was able to hydrolyse cellulose, starch, aesculin and Tween 80, but not egg yolk, gelatin, urea or casein. 16S rRNA gene sequence analysis demonstrated that this isolate was unique, showing only 88.4-91.0% sequence similarity to its closest neighbours, including members of the genera Glaciecola (88.4-91.0%), Alteromonas (88.7-89.6%), Aestuariibacter (89.3-90.4%), Salinimonas (89.0%), Bowmanella (90.1-90.3%) and Agarivorans (88.5-89.9%). Phylogenetic analyses demonstrated that strain YM01(T) formed a distinct clade closely related to species of the family Alteromonadaceae within the group of Alteromonas-like gammaproteobacteria. It contained menaquinone MK-7 as the predominant isoprenoid quinone and C(16:0) (38.3%), C(16:1)ω7c and/or iso-C(15:0) 2-OH (29.0%), C(18:1)ω7c (9.3%) and C(10:0) 3-OH (8.2%) as major cellular fatty acids. Phosphatidylethanolamine, phosphatidylglycerol and an aminophospholipid were the major phospholipid constituents. The DNA G+C content was 44.8 mol%. Based on its phenotypic, chemotaxonomic and phylogenetic distinctiveness, strain YM01(T) is considered to represent a novel species in a new genus in the Gammaproteobacteria, for which the name Catenovulum agarivorans gen. nov., sp. nov. is proposed; the type strain of Catenovulum agarivorans is YM01(T) (=CGMCC 1.10245(T) =DSM 23111(T) =JCM 16580(T)).