This report summarizes the results of a study on the life cycle of a member of the trematode family Acanthooolpidae, genus Stephanostomum Looss, 1899, which was carried on during the summers of 1948, 1949, 1951, and the winter of 1948-49 at the Marine Station of the College or the Pacific at Dillon Beach, California. Only one previous experimental work has been done on the life cycle of any member of this family. Martin (1939) reported the life cycle of Stephanostomum tenue (Linton) which he had obtained at Woods Hole, Massachusetts. Modifications of his technical procedures have been used in this study. It is hoped that this endeavor will further facilitate investigations in life cycle studies. The results obtained may be of economic value because the adult members of the family are parasitic in marine fishes, many of which are of commercial importance.