A number of marine bacteria isolated from young biofilms were characterized as belonging to the family Flavobacteriaceae. The taxonomic characterization of strain KOPRI 13649T, which was isolated from an acrylic surface at the seashore at Gangneung, Korea, is reported here. The nearly complete 16S rRNA gene sequence of strain KOPRI 13649T was determined and was found to have a high level of similarity with that of Psychroserpens burtonensis (95.0-95.6 %). In addition, phylogenetic analysis and comparison with closely related strains confirmed that the strain represented a novel member of the genus Psychroserpens. The major respiratory quinone of strain KOPRI 13649T was MK-6 and the DNA G+C content was 29.8 mol%. The dominant fatty acid methyl esters were i-15 : 0, a-15 : 0, i-16 : 0, i-15 : 1omega10, 16 : 1omega7 and 15 : 0. Growth was observed at 10-34 degrees C (optimum 30 degrees C), at pH 6-9 (optimum 6.5-8.0) and with 0.5-4 % NaCl (optimum 1 %). On the basis of the polyphasic taxonomic evidence presented, strain KOPRI 13649T (=KCCM 42261T=JCM 13413T) should be classified as the type strain of a novel species in the genus Psychroserpens, for which the name Psychroserpens mesophilus sp. nov. is proposed.