Electron microscopic examinations of ultrathin sections reveal the presence of spherical virus-like particles in the digestive gland, kidney, and the intestine of moribund scallop Chlamys farreri collected during a severe mortality in Jiaozhou Bay ( Qingdao , China) in 2000. The virions are approximately 130 to 170 nm in diameter and have a bilaminal envelope, while the nucleocapsids are 90 - 140 nm. The virus occurs and distributes in the cytoplasmic vesicles of infected cells, and it is arranged in a scattered form without occluding protein. The viral part icles are consistent in size and morphology in preparations derived from the isolation and also from ultrathin sections. The spikes of envelope are clearly observed with negative staining. By the same methods, this virus was not found in the healthy scallop, indicating that this virus might be the causative aetio logy for the abnormal mortality.