Following severe outbreaks of a disease with dermal necrosis, several bacterial strains were isolated from sea bass (Dicentrarchus labrax) reared on French coasts. A phenotypic study revealed that the isolates closely resembled Flexibacter maritimus. Nine selected strains were compared to 8 F. maritimus reference strains from other geographical origins and to the type strain NCIMB 2154. DNA base composition of the strains tested was in the G + C range of 30.6 to 23 mol%. A study on the DNA relatedness among the strains by the S1 nuclease DNA-DNA hybridization method showed that the 12 strains tested formed a tight genomic species with DNA homologies above 74%. The profiles obtained from API ZYM galleries, polyacrylamide gel electrophoresis of whole-cell proteins, and fatty acids analysis revealed that F. maritimus is a very homogeneous bacterial species. This is the first identification of this fish pathogen in France and its first isolation from sea bass.