Internal pathologic conditions in four stocks of Biomphalaria glabrata involve amebocytes. Most snails exhibiting these conditions are also nonsusceptible to infection with Schistosoma mansoni. Fine structure studies reveal that periaortic and atrial amebocytic accumulations show two responses that involve heart tissue: a generalized response in which amebocytes have infiltrated the myocardium and focal areas of nodules in which heart tissue is encapsulated and destroyed. In pericardial accumulations amebocytes are subspherical at the periphery with their numerous filopodia intertwining; centrally they elongate and flatten to form a nodule. Hemocoelic accumulations are similar to pericardial ones but more compact. Hyalinocytes and granulocytes were both found in the four types of amebocytic accumulations; each contained phagocytized material. Acid phosphatase reaction product was detected in the Golgi bodies, primary phagosomes, multivesicular bodies and myelin bodies of granulocytes.