Two mercury-resistant strains of heterotrophic, aerobic, marine bacteria, designated AT1(T) and AS1(T), were isolated from water samples collected from the Er-Jen River estuary, Tainan, Taiwan. Cells were Gram-negative rods that were motile by means of a single polar flagellum. Buds and prosthecae were produced. The two isolates required NaCl for growth and grew optimally at about 30 degrees C, 2-4 % NaCl and pH 7-8. They grew aerobically and were incapable of anaerobic growth by fermenting glucose or other carbohydrates. They grew and expressed Hg(2+)-reducing activity in liquid media containing HgCl(2). Strain AS1(T) reduced nitrate to nitrite. The predominant isoprenoid quinone was Q(8) (91.3-99.9 %). The polar lipids of strain AT1(T) consisted of phosphatidylethanolamine (46.6 %), phosphatidylglycerol (28.9 %) and sulfolipid (24.5 %), whereas those of AS1(T) comprised phosphatidylethanolamine (48.2 %) and phosphatidylglycerol (51.8 %). The two isolates contained C(16 : 1)omega7c and/or iso-C(15 : 0) 2-OH (22.4-33.7 %), C(16 : 0) (19.0-22.7 %) and C(18 : 1)omega7c (11.3-11.7 %) as the major fatty acids. Strains AT1(T) and AS1(T) had DNA G+C contents of 43.1 and 45.3 mol%, respectively. Phylogeny based on 16S rRNA gene sequences, together with data from morphological, physiological and chemotaxonomic characterization, indicated that the two isolates could be classified as representatives of two novel species in the genus Alteromonas, for which the names Alteromonas tagae sp. nov. (type strain AT1(T)=BCRC 17571(T)=JCM 13895(T)) and Alteromonas simiduii sp. nov. (type strain AS1(T)=BCRC 17572(T)=JCM 13896(T)) are proposed.