Two facultatively anaerobic, nitrogen-fixing bacteria (strains MSSRF30(T) and MSSRF31) were isolated from a mangrove-associated wild rice (Porteresia coarctata Tateoka). These strains were determined to be nitrogen-fixers using the acetylene reduction assay and by PCR detection of a nifH gene amplicon. Phylogenetic analyses based on 16S rRNA gene sequences indicated that the novel strains were most closely related to Vibrio fluvialis LMG 7894(T) (96.8 % gene sequence similarity), Vibrio furnissii LMG 7910(T) (96.8 % sequence similarity) and Vibrio tubiashii CIP 102760(T) (96.7 % sequence similarity). Further multilocus sequence analysis using recA, pyrH, rpoA and nifH genes also showed low levels of sequence similarities (83-93 %) with all species of the genus Vibrio with validly published names. A multigene phylogenetic tree using concatenated sequences of the four genes (16S rRNA, rpoA, recA and pyrH) showed that strains MSSRF30(T) and MSSRF31 occupied a distinct phylogenetic position, forming a long branching that was not clustered with any other recognized Vibrio species. The fatty acid profile also suggested that the novel strains belonged to the genus Vibrio. The results of physiological and biochemical tests, genomic fingerprinting and DNA-DNA hybridization analyses clearly differentiated both novel strains from their closest phylogenetic neighbours, Vibrio cholerae IID6019, Vibrio mimicus LMG 7896(T), V. fluvialis LMG 7894(T) and V. furnissii LMG 7910(T). Several phenotypic traits enabled the differentiation of strain MSSRF30(T) from other species of the genus Vibrio. The DNA G+C content of strain MSSRF30(T) was 44.4+/-3.1 mol%. Based on genotypic, phenotypic, chemotaxonomic, phylogenetic and DNA-DNA hybridization analyses, the name Vibrio porteresiae sp. nov. (type strain MSSRF30(T)=LMG 24061(T)=DSM 19223(T)) is proposed for this novel taxon.