On the basis of phenotypic, genotypic characteristics and analysis of 16S rRNA gene sequences, a novel species belonging to the genus Alteromonas is described. A non-pigmented, motile, Gram-negative bacterium designated R10SW13(T) was isolated from sea water samples collected in Chazhma Bay (Sea of Japan, Pacific Ocean). The novel organism mainly grew between 4 degrees C and 37 degrees C, was neutrophilic and slightly halophilic, tolerating up to 10 % NaCl. Strain R10SW13(T) was haemolytic and was able to degrade starch and Tween 80 and to degrade gelatin and agar weakly, but did not degrade casein. Phosphatidylethanolamine (44.3 +/- 0.9 %) and phosphatidylglycerol (55.7 +/ -0.9 %) were the predominant phospholipids. The major fatty acids formed were typical for the genus Alteromonas, including 16 : 0, 16 : 1omega-7 and 18 : 1omega-7. The G + C content of the DNA was 43.4 mol%. DNA-DNA hybridization experiments showed 38-53 % binding with the DNAs of type strains of phylogenetically related species of the genus Alteromonas, namely: Alteromonas macleodii, Alteromonas marina, Alteromonas stellipolaris, Alteromonas litorea, 'Alteromonas macleodii subsp. fijiensis' and 'Alteromonas infernus'. Based on these results, a novel species, Alteromonas addita sp. nov., is proposed, with strain R10SW13(T) (=KMM 3600(T) = KCTC 12195(T) = LMG 22532(T)) as the type strain.