Tissue reactions in the marine gastropods Cerithium moniliferum Kiener and Peristernia australiensis Reeve infected with the aspidogastrid Lobatostoma manteri are described. The former species harbours usually a single parasite coiled up in a cavity formed by the main and one (or some?) side ducts of the digestive gland, with metaplasia of the duct epithelium, hyperplasia of the interfollicular connective tissue and amoebocytes, and necrosis of some glandular follicles; the latter harbours up to six parasites in the stomach and large ducts of the digestive gland, with a thickening of the subepithelial connective tissue layer. There was a strong decrease in the relative number of infected snails of both species from January 1971 to April 1972. During the period of high frequency of infection, Cerithium infected with Digenea contained Lobatostoma relatively more frequently than Cerithium without Digenea. Snails with double infections disappeared first. Infection with Lobatostoma did not affect the relative number of egg - producing Cerithium during the period of high frequency of infection. Worms from different snail hosts have the same number of marginal alveoli but a different maximum body length. The size of ovary and testis (and perhaps cirrus pouch) is increased in specimens from fish. The relative sizes of other organs examined are not or only negligibly affected by the hosts. Pressing of specimens does not affect slope and intercept of the regression lines for most organs, except those for the pharynx, which have the same slope but different intercepts.