Abstract: The bay scallop Argopecten irridians was introduced into China in 1982 and has been the important cultured scallop from then. The epizootiological investigation was carried out from November, 1999 to April of 2001 and a chlamydia-like organism was found in the epithelium of digestive diverticular. Samples of A. irridians in different stage were collected from Jincheng culture facility on Laizhou Bay, Laizhou County, Shandong, China. The sampling interval was 1 month. For light microscopy, the tissues including mantle, gill, digestive gland, intestine and kidney were excised and fixed with Bouin's fixative. All tissues were dehydrated through an ascending ethanol series, embedded in paraffin, sectioned at 5μm and stained with haematoxylin and eosin (H&E). For electron microscopy, fresh tissues were cut at 0.5—1mm3, fixed in 2.5% glutaradehyde in phosphate buffer system (pH7.4) at 4℃ and post fixed in 1% Osmium tetraoxide for 1 hour, dehydrated in an ethanol series, embedded in Epon-812. Ultrathin section were cut on LKB ultramicrotome, stained with uranyl acetate and lead citrate and examined with H-7000 transmission electron microscope at 75kV. The light examination on histological sections of different tissues showed that basophilic intracellular inclusions were only present in the epithelium of the digestive diverticular. The inclusions take the shape of round or ellipsoid with the size of 9μm×7μm. TEM examination of ultrathin section of tissues revealed that the intracellular basophilic inclusions were the clone of prokaryotes, which was membrane limited in vacuoles. The prokaryote was of three morphologies of different sizes and structures. The larger body was reticulate body which was round, ellipsoid or irregular shaped, and the size was 890.5(±164.6)nm×623.6(±129.3)nm(n=20). The double membrane-bound structure can be clearly seen. The outer membrane was rippled with a slime layer around it. In the central area it was electron-lucent with fine filament structure, in the peripheral area it was ribosome-like granular electron-dense material. The binary fission can be observed. The smaller body was elementary body that was short-rod or spindle shaped with the size of 317.5 (±40.1)nm×180.3(±40.0)nm(n=18). The cell wall of double trilamina structure was very clear. The central area was electron-dense. There was an evident space area between cell wall and central area. The middle-sized body was intermediate body, the size of which was between that of reticulate body and elementary body, the cell wall was similar to that of elementary body and the inner structure was similar to that of reticulate body. Intermediate body may be the transitional stage from reticulate body to elementary body. Besides three morphologies, there were also some small no structure blebs in the vacuole. The host cell response was not evident besides of hypertrophy. With the vacuole growing up, it took up the most space of host cell and the organelles were pressed to the side. The characteristics of the prokaryotes revealed that it should be categorize into chlamydia-like organism (CLO) and the size and morphology suggested that it was not the same species as reported in other marine bivalves. The present study was the first report of chlamydia-like organism in cultured bay scallop A. irridians in China. Although there was not obvious observed damages at present, the potential effect of chlamydia-like organism on the host need to be paid more attention to because of the importance of bay scallop culture to Chinese mariculture.