The scallop, Chlamys farreri , is one of the major species cultured in North China, and its culture in commercial scale has been performed for more than 20 years. The highest production of about 1 million metric tons has been achieved in 1996. The great expansion and intensif ication induced the occurrence of disease since 1990. s, and became epizootics f rom 1997. It is believed that the disease so called -Scallop Acute Viral Necrot ic Disease. ( SAVND) has becoming the major limiting factor in the development of the industry, and has hindered the economic process in the coast area of the region. The earlier investigation from our research group showed that the disease occurred in the second-year-old scallops, which had an averaged shell height of 5. 0 ? 0. 9 cm in Jiaozhou Bay. The mortality f irstly occurred in the early July and peaked in the end of the month, as the temperature reached the highest period ( 25- 26. 7bC) in the year. Subsequently, the disease subsided in the early August and the cumulative mortality could be above 90% . In most cases, the morbidity was acute and the course of disease was only about one week. In order to make sure the main pathogen of this disease, an intensive investigation was undertaken recently. Firstly, this paper is to report histopathological and microbiological evidence of the cultured scallop on a diseased condition, and described a possible virus-like etiology causing the massive death of the scallop. Electron microscopic examinations of ultrathin sect ions reveal the presence of spherical virus-like particles in the digestive gland, mantle, kidney, and the intestine of moribund scallop Chlamys farreri collected from 6 epidemic areas during a severe mortality on the coast of Shandong ( China) in 2000- 2002. Relevant pathologic change can be observed. Virions are approximately 130 to 170 nm in diameter and have a bilaminal envelope, while the nucleocapsids are 90- 140 nm. Spike of envelope, observed by negative stain after isolation and purif ication, are 20nm in length. The virus occurs and assembles in the cytoplasmic vesicles without occlusion. Next , art if icial infective experiments and back-check of infective samples were taken to confirm whether the scallop disease was aroused by the virions. Artificial infective experiment s of healthy scallop were carried out by injection and by waterborne of viral preparation isolated from moribund scallop of epidemic area. The results showed that the injection and waterborne administration had high mortality of 75% and 68. 7%, respectively. Meanwhile, the control had 87. 5% survival rate. The symptom appeared after artificial inoculation was similar to those shown by diseased scallop under natural conditions. Moreover, the virions could be detected in infected cell of diseased scallop in the infective experiment by electron microscopy, in which the morphological and pathological characters were ident ical with those formed in diseased scallop under natural conditions. These results strongly support the conclusion that virus is the causative aet iology for the abnormal mortality.