Histochemical studies of the digestive gland of Lymnaea truncatula have revealed features of the normal digestive processes in which at least 3 cell types are involved: digestive cells, mucus cells and basiphil cells. An intracellular vacuolar digestive system containing lysosomal enzymes appears to operate in the digestive cells. Parasitization by Fasciola hepatica evokes considerable structural and histochemical changes in the host digestive gland. There is, in general, increased cellular vacuolation together with increased lysosomal enzyme activity. The overall effect appears to be an increase in intracellular digestive processes, including autolysis, and it is suggested that this may be a consequence of the host attempting to maintain its nutritional requirements under parasitic stress. Starvation of un-infected specimens induces structural and histochemical changes which closely resemble the effects of parasitization.