Light and electron microscopical studies were carried out to characterize the hemocytes of the oyster, Crassostrea ariakensis. Three types of hemocytes were recognized: granulocytes, large hyalinocytes and small hyalinocytes. The large hyalinocytes and small hyalinocytes were agranular cells. The large hyalinocytes presented thin cytoplasm and high nucleus: cytoplasm ratios. The small hyalinocytes were the most homogeneous in shape and showed round or oval. They have the highest nucleus: cytoplasm ratios and contained very thin cytoplasm. The granulocytes showed abundant cytoplasm and low nucleus: cytoplasm ratios. At the ultrastructural level, the large hyalinocytes cytoplasm contained a variable number of mitochondria, Golgi complex and endoplasmic reticulum. They showed a total absence of cytoplasmic granules or a few small electron-lucid vesicles of different sizes. The granulocytes showed more polymorphic than the agranular cells, with numerous pseudopodia sprouting off their surface. The granulocytes showed similar organelles, but on the contrary they had abundant electron-dense particles or electron-lucent granules in the cell cytoplasm. The small hyalinocytes presented a total absence of cytoplasmic granules and few organelles and only one or two mitochondria were sometimes observed in the cytoplasm. Total hemocyte counts gave a mean (±SE) concentration of 2.06 ± 0.20 × 107 cells mL−1 of hemolymph. Differential hemocyte counts identified the granulocytes as the predominant cell type, followed by the large hyalinocytes and the small hyalinocytes. The percentages of the three cell types in the hemocyte population of C. ariakensis were 68.4%, 21.6% and 9.0%, respectively. The quantitative study of phagocytosis showed that the granulocytes were phagocytic cells and the agranular cells showed a limited phagocytic ability. After the separation in Percoll density gradient, the granulocytes were separated from agranulocytes and pure granulocytes were obtained. However, the separation of different agranular cell types (large hyalinocytes and small hyalinocytes) was not achieved by Percoll or Ficoll gradient centrifugation. Our study provides a morphofunctional basis for the cellular defense mechanisms in C. ariakensis.