The morphology and ultrastructure of the blood cells of the freshwater snails Lymnaea stagnalis, Biomphalaria glabrata, and Bulinus truncatus were studied. By performing in vitro experiments and enzyme histochemical studies, special attention was paid to the role of the blood cells in phagocytosis of foreign particles. No fundamental differences were found in the ultrastructure, lysosomal enzyme contents, and phagocytic capacities of the blood cells of these species. It is concluded that only one type of blood cell, the amoebocyte, exists in the freshwater snails. Amoebocytes constitute a morphologically and functionally heterogeneous population of cells, ranging from round (electron-dense) cells with the morphological characteristics of young cells to highly phagocytic spreading cells with a prominent lysosomal system. In addition to acid phosphatase, nonspecific esterase and peroxidase were found within the lysosomes. The presence of enzyme activity in the RER and the Golgi bodies indicates that amoebocytes are able to synthesize lysosomal enzymes continuously.