Ameel (1934) worked out the life cycle of Paragonimus kellicotti in experimental infections of Pomatiopsis lapidaria. Later, Chen (1937) made a study of the germ cell cycle of this species in which she followed the germinal cells in sections through all the stages of development. This was the most complete study of its type that had been made up to that time, and gave strong support to the theory that development in the germinal sacs of the digenetic trematodes is a germinal lineage. She determined that the diploid number of chromosomes was 16 and found no reduction in the divisions of germinal cells in the germinal sacs. This observation has been confirmed in the present study. During the summers of 1948, 1949 and 1950 at the University of Michigan Biological Station we studied the development of the mother sporocyst and rediae of P. kellicotti and traced the cells of the germinal line in living material. Our studies confirm most of the observations of Ameel and Chen on these stages and add new information on certain phases of their development. The study of living material gave a much clearer picture of the morphological relations of the stages in germinal development than could be obtained from sections.