A rare kind of parasitic turbellarian, parasitizing Sciaenops ocellatus, causing serious gill damages, and causing high mortality of the host fish, was reported for the first time by our previous study at Luoyuan Bay in October 2000. The subsequent survey revealed that this disease had the characters of large scale, high incidence and high intensity. In 2002 this disease took place in a company at Zhangpu County of Fujian Province, where Paralichthys olivaceus was cultured. It was not diagnosed and controlled immediately. The infection rate was over 50%. The mortality ranged from 20% to 50%. The loss was approximately 40 million yuan. This disease also caused the death of as many as twenty thousand Pseudosciaena croceas within a week at net cages of No. 350 cultured in Luoyuan Bay. The mortality was higher than 50%. Therefore,it is urgent to study the epidemiology of this disease and to discover the method to control it. The biological characteristics of this turbellarian were studied by field survey, biostatistics, experimental infection, observation of living specimen, histological method, and so on. The results showed that the disease prevailed all over the cultural sea area in eastem Fujian, including Luoyuan County, Ningde City, Lianjiang County and Xiapu County, and southem Fujian, including Zhangpu County, Xiamen City and Tong＇ an County. This kind of turbellarian was found in both the cultural cages and the cultural tanks. It infected not only S. ocellatus but also P. crocea, Miichthys miiuy, P. olivaceus, Sparus macrocephalus and Takifugu oblongus. These mrbellarians parasitized the gill, fins and the body surface of many kinds of marine cultured fishes. They made great lesion to the place where they lodged, causing asphyxiation or the secondary infection of bacteria which resulted in a high mortality of 20% - 60%. This disease prevailed mainly in autumn, from August to November, at the water temperature of 22 - 26 ℃ and the salinity of 25 - 30. A coincidence of high incidence area and the nearby seashell culture area, such as abalone and oyster culture area, was found. The primary experimental infections showed the cornmensul associations of the turbellarian with clam and oyster. However, it needs further confirmation on which kind of mollusc could serve as the reservoir host of the turbellarian. This study could provide scientific bases to control the diseases. According to the results of survey and experiments, several treatments to the disease were provided. Cupric sulfate, formaldehyde, trichlorphon can have a certain curative measure by spilling, bathing and hanging bag in the water. The using of underwater, reducing salinity and temperature, improving nutrition and using medicine reasonably and so on were also proved to be effective in the control of the disease.