Light, transmission and scanning electron microscopical studies were carried out to characterize the hemocytes of the scallop Chlamys farreri (Jones & Preston). Five types of hemocytes were recognized: type 1 small hyalinocytes (2.44 ± 0.11 μm, 45% to 50%), type 2 large hyalinocytes (4.83 ± 0.28 μm, 15–20%), type 3 small granulocytes (4.07 ± 0.15 μm, 15% to 20%), type 4 medium granulocytes (7.20 ± 0.26 μm, 20–25%) and type 5 large granulocytes (13.87 ± 0.73 μm, 3% to 5%). Granulocytes showed larger sizes and smaller N/C ratios than hyalinocytes. The mean hemocyte concentration was about (3.03 ± 0.11) × 10⁷ cells mL⁻¹. Among hemocytes, 42.6% are granular cells and 57.4% are agranular cells. These gave a relatively systematic classification scheme for hemocytes of Chlamys farreri. Three kinds of granules were identified: high electron-dense granules, low electron-dense granules and medium electron-dense granules based on TEM studies. Both granulocytes and hyalinocytes showed phagocytic response to the two strains of bacteria, E. coli and Rickettsia-like organisms. The phagocytic ability of granulocyte was significantly higher (30% to 40%) than that of hyalinocyte (4.8% to 14%).