As tidal flats have sharply decreased during this half century in Japan, the organisms living mainly in the habitat are on the crisis of extinction. So we planned to recognize the present condition of the snails and their trematode parasites in tidal flats. We surveyed many tidal flats in Japan. In the Sanbanse, the Fujimae flat, the Shimanto River, the Kakise River and the Kokuba River, the snails were hardly collected. In the Yoshino River snails were living on the very restricted area, and the infected snails were not detected. In the Sone flat many snails belonging to four species are living on the flat, but no snails were infected by any cercariae. In the four tidal flats in Kagawa Pref. five species of cercaria were detected from three species of snails. Especially in the Doki River Cerithidea rhizophorarum were parasitized by two species of cercariae with high rate, but co-infection rates of two species were significantly lower than the expectation value when two species of cercaria infect to the snail independently. It is concluded that two species of ceracaria infect the snails avoiding each other, but not completed exclusive. In the Awase tidal flat four species of snails were infected by cercariae. And in the Urauchi River two species of snails were coexist sympatric in the restricted area, but the parasite rates were significantly different between snail species. It is concluded that the one species were preferred to the other. Snails and cercariae living in the tidal flats in Japan were decreased more than we expected. However, in Kagawa Pref. and Okinawa Pref. the tidal flats maintained the diversity of organisms.