The temperate sea cucumber species, Apostichopus joponicus, naturally distributed in Bohai Bay and Yellow Sea, is a valuable one due to its nutritional and medicinal properties. In recent years, sea cucumber aquaculture has developed rapidly along the northern coast of China, while the expansion and intensification of A. japonicus farming has led to the occurrence of some diseases. In the spring of 2003, the cultured sea cucumber suffered from serious disease in many farms along the coast of Qingdao, Shandong Province, which was infectious and lethal, causing 30% losses around the area. The symptom displayed as anorexia, shaking head, mouth tumidity, viscera ejection, general atrophy, and skin ulceration, the disease was tentatively termed as ＂skin ulcerative syndrome＂. The bacterium KL-1 was isolated from the focal area of the infected sea cucumber, and it was dominant amongst the isolates. Artificial infection test proved that KL-1 was the causative pathogen associated with the disease. The bacterium KL-1 was identified as Vibrio splerdidus, by the means of morphological, physiological and biochemical tests, and 16S rDNA sequence analysis. This paper revealed for the fisrst time that the causative pathogen leading to the mass mortality of the sea cucumber in the area, which will be helpful in the disease control and health management during sea cucumber cultivation.