A Gram-negative, short rod to oval-shaped bacterium (strain MW2a(T)) was isolated from the surface of leaves of the black mangrove Avicennia germinans and subjected to a polyphasic taxonomic study. Strain MW2a(T) was moderately halophilic, growing at NaCl concentrations in the range 0-25 % (w/v) with optimum growth at 5 % (w/v) NaCl. Growth occurred at 12-40 degrees C (optimum, 30-35 degrees C) and at pH 5.0-9.0 (optimum, pH 7.0-8.0). Strain MW2a(T) was strictly aerobic. Phylogenetic analysis based on the 16S rRNA gene showed that the strain belongs to the genus Halomonas. The closest relative was Halomonas marisflavi, with 98.6 % 16S rRNA gene sequence similarity. The DNA G+C content of strain MW2a(T) was 61.5 mol%, which is in the range of values for Halomonas species. DNA-DNA hybridization with H. marisflavi showed a relatedness of 42 % and lower values were obtained with respect to other related Halomonas species. The major fatty acids were C(16 : 0), C(19 : 0) cyclo omega8c, C(18 : 1)omega7c and C(12 : 0) 3-OH. Overall, the phenotypic, genotypic and phylogenetic results presented in this study demonstrate that strain MW2a(T) represents a novel species within the genus Halomonas. The name Halomonas avicenniae sp. nov. is proposed, with strain MW2a(T) (=CECT 7193(T)=CCM 7396(T)) as the type strain.