Technical Abstract: Shewanella marisflavi isolate AP629 was characterized as a novel pathogen of sea cucumber. The LD50 values (14 days) in sea cucumber and swordtail fish were 3.89 × 106 and 4.85 × 104 CFU g-1 body weight, respectively. Studies on S. marisflavi had been conducted, including morphology, physiological and biochemical characteristics, hemolysis, whole-cell protein and 16S rRNA gene sequence. Colonies of S. marisflavi appeared faintly red on MA and green on TCBS medium. S. marisflavi had polar flagella. The cells were gram-negative, oxidase- and catalase-positive and not sensitive to O/129. The bacterium exhibited ß-hemolysis on sheep blood agar and produced H2S. S. marisflavi survived and grew at temperature 4-35 ºC, pH 6.0-9.2, and in the presence of 0-8% NaCl. The whole-cell proteins included 13 discrete bands and proteins of molecular weight 87 kDa, 44 kDa and 39 kDa existed in all five strains of Shewanella spp. The difference in 16S rRNA gene sequences occurred at nucleotide 446 bp: S. marisflavi (KCCM 41822) - G, isolate AP629 - A, respectively. To our knowledge, this is the first report that Shewanella showed pathogenicity to sea cucumber. In respect of potential pathogenicity of Shewanella spp. to human and fish, more attentions should be paid in the handling of sea cucumber.