The agar-degrading bacterium GNUM-1 was isolated from the brown algal species Sargassum serratifolium, which was obtained from the West Sea of Korea, by using the selective artificial seawater agar plate. The cells were Gram-negative, 0.5-0.6 ?m wide and 2.0-2.5 ?m long curved rods with a single polar flagellum, forming nonpigmented, circular, smooth colonies. Cells grew at 20oC- 37oC, between pH 5.0 and 9.0, and at 1-10% (w/v) NaCl. The DNA G+C content of the GNUM-1 strain was 45.5 mol%. The 16S rRNA sequence of the GNUM-1 was very similar to those of Alteromonas stellipolaris LMG 21861 (99.86% sequence homology) and Alteromonas addita R10SW13 T (99.64% sequence homology), which led us to assign it to the genus Alteromonas. It showed positive activities for agarase, amylase, gelatinase, alkaline phosphatase, esterase (C8), lipase (C14), leucine arylamidase, valine arylamidase, α-chymotrypsin, acid phosphatase, naphthol- AS-BI-phosphohydrolase, α-galactosidase, β-galactosidase, β-glucosidase, catalase, and urease. It can utilize citrate, malic acid, and trisodium citrate. The major fatty acids were summed feature 3 (21.5%, comprising C16:1ω7c/iso- C15:0 2-OH) and C16:0 (15.04%). On the basis of the variations in many biochemical characteristics, GNUM-1 was considered as unique and thus was named Alteromonas sp. GNUM-1. It produced the highest agarase activity in modified ASW medium containing 0.4% sucrose, but lower activity in rich media despite superior growth, implying that agarase production is tightly regulated and repressed in a rich nutrient condition. The 30 kDa protein with agarase activity was identified by zymography, and this report serves as the very first account of such a protein in the genus Alteromonas.