Bivalve molluscs possess several types of hemolymph cells (hemocytes), involved in internal defense and other physiological functions. Studies in pathological and experimental conditions have shown their micro- and macrophagic ability. However, the mechanisms of intracellular digestion are incompletely known in these organisms. The lysosomal apparatus of hemocytes was therefore investigated in several species by hydrolase localization with the electron microscope. The cytoplasmic granules of granulocytes displayed a strong acid beta -glycerophosphatase activity. Previous observations had shown that these granules may contain phagocytized material. This indicates that intracellular digestion is achieved in these organelles. Acid beta -glycerophosphatase activity was also observed in the Golgi apparatus and adjacent secretory vesicles revealing a role in granule formation. The heterogeneity in hydrolase distribution among the cell types suggested functional peculiarities.