Light,electron microscopic and flow cytometer studies were conducted to characterize morphology and structure and phagocytosis of haemocytes in blood clam Tegillarca granosa. Based on cell size,and microstructure and granular types of cell,blood clam haemocytes were divided into red granulocyte(89.67%),basophil granulocyte(7.05%)and hyalinocyte(3.28%)respectively under light microscopy.The haemocyte densities in healthy blood clam haemolymph were assessed(3.29±0.82)×10⁶ cell/mL.Three kinds of haemocytes were respectively recognized by transmission electron microscope:granulocyte type Ⅰ,granulocyte type Ⅱ and agranulocyte.It was observed under scanning electron microscopy that haemocytes appear smooth-surface and some of them have knot-like projections in cell surface.APIZYM assays on enzymatic activities of 19 enzymes in haemolymph were performed and,of these,13 and 10 enzymes were detected respectively in serum and cell.There were marked increases in the levels of ALP,ACP,ELIP(C8)and AGLA in serum and haemocytes after being stimulated by zymosan.Observation of phagocytosis in vitro revealed that there was phagocytic response in red granulocyte to zymosan and there was no significant phagocytic response in basophil granulocyte and hyalinocyte.It was observed that basophilic granulocyte accumulation and hyalinocyte adsorption after being stimulated by Aeromonas hydrophila.The study of environmental stress factors on phagocytosis activity revealed that the temperature and salinity have significant influence on phagocytosis activity.The phagocytosis percentage of blood clam reached the highest values with optimum growth temperature(13-30 ℃)and salinity(20.0-26.2)×10^-12.