This study deals with the cercarial fauna of marine mollusks in the littoral zone and estuaries of the California coast between Duxbury Reef and Fort Ross. The investigation began in June, 1968 at the Pacific Marine Station, Dillon Beach, California and continued through July, 1969, with monthly collections at various localities (see map). Two major objectives were aimed at: first, to continue a survey of the cercariae of the area begun by Fisk (194) and second, to establish a basis for future life history studies.