Sporocycts of the "pygmaeus" microphallides (Microphallus piriformes) are localized in hepatopancreas and gonads of Littorina molluscs causing total parasitic castration. A histological study of penial glands in Littorina saxatilis and L. obtusata males infested with trematodes M. piriformes has been made. Copulatory organs of noninfested molluscs, molluscs after recent contamination (with not completely formed daughter sporocysts), and molluscs containing mature metacercariae inside daughter sporocysts were examined. Based on the data obtained, probable dynamics of the histological structure of infested glandular apparatus was established. It was shown, that the trematode infestation have an influence on the muscular and secretory parts of penial glands. The wall of the penial gland muscular capsule becomes more fine in infested L. saxatilis. On the contrary, this wall is vastly thicker in infested L. obtusata, as compared with noninfested individuals. Glandular cells of the molluscs' penial glands decreases the amount of granular secret in both species. In L. obtusata the number of secretory cells is shown to be reduced up to their total disappearance. The above pathological changes probably prevent normal function of penial glands.