Hemocytes represent one of the most important defense mechanisms against foreign material in Mollusca. The morphology, hematological parameters and behaviour of hemolymph cells were studied in the southern quahog Mercenaria campechiensis, the eastern oyster Crassostrea virginica, and the blood ark Anadara ovalis challenged with the bacteria Vibrio vulnificus and V. anguillarum. Two general classes of hemocytes (granular and agranular) exist in C. virginica and M. campechiensis. In contrast, A. ovalis possesses 3 general classes (granular, agranular and erythrocytes). Three types of granules were identified by light microscopy. When hemolymph cells were studied by transmission electron microscopy, the cytoplasm of hemolymph cells was noted to contain many organelles, including electron dense granules. Both agranular and granular hemolymph cells were capable of colchicine-sensitive pseudopodial movement and spreading. The results indicate that marine bivalves possess hemolymph blood cells which may play a role in the internal defense paralleling mammalian phagocytes. The morphology of these cells, as determined by light, scanning and transmission electron microscopy, showed some similarity to mammalianmononuclear phagocytes. The sub-cellular events of molluscan hemocyte phagocytosis of V.vulnificus and V~ anguillarum were studied by both scanning and transmission electron microscopy. The role of these cells and the factors which govern their behavior are of economic and public health importance.