Bay scallop is one of the main culture species in China .But in recent years , a disease frequently occurred on cultured scallop , and led to a serious loss .In spring of 2001 , a mantle erosion disease of cultured scallop Argopecten irradians Lamarck broke out in Qingdao , Shangdong Province , which led to death of 50 % parent scallop .It was first found in indoor cultured parent scallop in March .From April to May , the scallop cultured in sea area took on the similar symptoms .With the rise of water temperature the death rate increased .The main clinic symptoms of diseased scallop are the erosion of the mantle , and visceral mass thin .To the serious ones , about 2/3 of the diseased scallop' s mantle ulcerated like glue-water , digestive diverticula became loose , gonad shrinked , adductor had no force to open or close , gills slightly ulcerated .Vivisecting the diseased scallop , fixing the mantle , gill , digestive diverticula , gonad and etc .in 2 .5 % glutaraldehyde , and post fixing in 1 % osmium tetroxide to prepare electron microscopy specimen .Two parasitic micro-organisms were found in diseased scallop by the transmission electron microscopy .The virus is mainly found in digestive diverticula epithelium and connective tissue cytoplasm.The present paper reports the morphology , morphogenesis and cytopathology of the spherical virus .The mature virus is approximately 150 -180nm in diameter and possess a bilaminal envelopes .The formation and multiplication process of the virus occurred in lysosome near the nucleus .In the beginning , the myeloid body was formed in lysosome , and with the process of the virus formation the layers of the myeloid body increased and formed polymorphological virogenic stromas .The proteinic crystal structure was found in the virogenic stroma , and is a regular square structure .It is the initial report on mussel in the world .At post development of the process , the myeloid body disappeared , and many virus particles were filled in lysosome .As a definite feature in cells infected by the virus , most organelles were damaged , e .g .the endoplasmic reticulum was swelling and a lot of ribosomes detaching from it .The mitochondria was swelling , deforming and the crest were dissolving .The nuclear membranes were swelling or dissolving , and the amount of lysosome increased .No virus but many Chlamydia-like organisms were found in the epithelial cells of mantle and gonad .The result of observation showed that the virus and the Chlamydia-like organisms were correlated with the disease of scallop .